Source: parser.go in package go/parser

Package parser implements a parser for Go source files. Input may be provided in a variety of forms (see the various Parse* functions); the output is an abstract syntax tree (AST) representing the Go source. The parser is invoked through one of the Parse* functions. The parser accepts a larger language than is syntactically permitted by the Go spec, for simplicity, and for improved robustness in the presence of syntax errors. For instance, in method declarations, the receiver is treated like an ordinary parameter list and thus may contain multiple entries where the spec permits exactly one. Consequently, the corresponding field in the AST (ast.FuncDecl.Recv) field is not restricted to one entry.

package parser

import (
	"fmt"
	"go/ast"
	"go/scanner"
	"go/token"
	"strconv"
	"strings"
	"unicode"
)

The parser structure holds the parser's internal state.

type parser struct {
	file    *token.File
	errors  scanner.ErrorList
	scanner scanner.Scanner

Tracing/debugging

	mode   Mode // parsing mode
	trace  bool // == (mode & Trace != 0)
	indent int  // indentation used for tracing output

Comments

	comments    []*ast.CommentGroup
	leadComment *ast.CommentGroup // last lead comment
	lineComment *ast.CommentGroup // last line comment

Next token

	pos token.Pos   // token position
	tok token.Token // one token look-ahead
	lit string      // token literal

Error recovery (used to limit the number of calls to parser.advance w/o making scanning progress - avoids potential endless loops across multiple parser functions during error recovery)

	syncPos token.Pos // last synchronization position
	syncCnt int       // number of parser.advance calls without progress

Non-syntactic parser control

	exprLev int  // < 0: in control clause, >= 0: in expression
	inRhs   bool // if set, the parser is parsing a rhs expression

Ordinary identifier scopes

	pkgScope   *ast.Scope        // pkgScope.Outer == nil
	topScope   *ast.Scope        // top-most scope; may be pkgScope
	unresolved []*ast.Ident      // unresolved identifiers
	imports    []*ast.ImportSpec // list of imports

Label scopes (maintained by open/close LabelScope)

	labelScope  *ast.Scope     // label scope for current function
	targetStack [][]*ast.Ident // stack of unresolved labels
}

func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mode) {
	p.file = fset.AddFile(filename, -1, len(src))
	var m scanner.Mode
	if mode&ParseComments != 0 {
		m = scanner.ScanComments
	}
	eh := func(pos token.Position, msg string) { p.errors.Add(pos, msg) }
	p.scanner.Init(p.file, src, eh, m)

	p.mode = mode
	p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)

	p.next()
}

---------------------------------------------------------------------------- Scoping support


func (p *parser) openScope() {
	p.topScope = ast.NewScope(p.topScope)
}

func (p *parser) closeScope() {
	p.topScope = p.topScope.Outer
}

func (p *parser) openLabelScope() {
	p.labelScope = ast.NewScope(p.labelScope)
	p.targetStack = append(p.targetStack, nil)
}

resolve labels

	n := len(p.targetStack) - 1
	scope := p.labelScope
	for _, ident := range p.targetStack[n] {
		ident.Obj = scope.Lookup(ident.Name)
		if ident.Obj == nil && p.mode&DeclarationErrors != 0 {
			p.error(ident.Pos(), fmt.Sprintf("label %s undefined", ident.Name))
		}

pop label scope

	p.targetStack = p.targetStack[0:n]
	p.labelScope = p.labelScope.Outer
}

func (p *parser) declare(decl, data interface{}, scope *ast.Scope, kind ast.ObjKind, idents ...*ast.Ident) {
	for _, ident := range idents {
		assert(ident.Obj == nil, "identifier already declared or resolved")

remember the corresponding declaration for redeclaration errors and global variable resolution/typechecking phase

		obj.Decl = decl
		obj.Data = data
		ident.Obj = obj
		if ident.Name != "_" {
			if alt := scope.Insert(obj); alt != nil && p.mode&DeclarationErrors != 0 {
				prevDecl := ""
				if pos := alt.Pos(); pos.IsValid() {
					prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.file.Position(pos))
				}
				p.error(ident.Pos(), fmt.Sprintf("%s redeclared in this block%s", ident.Name, prevDecl))
			}
		}
	}
}

Go spec: A short variable declaration may redeclare variables provided they were originally declared in the same block with the same type, and at least one of the non-blank variables is new.

	n := 0 // number of new variables
	for _, x := range list {
		if ident, isIdent := x.(*ast.Ident); isIdent {
			assert(ident.Obj == nil, "identifier already declared or resolved")

remember corresponding assignment for other tools

			obj.Decl = decl
			ident.Obj = obj
			if ident.Name != "_" {
				if alt := p.topScope.Insert(obj); alt != nil {
					ident.Obj = alt // redeclaration
				} else {
					n++ // new declaration
				}
			}
		} else {
			p.errorExpected(x.Pos(), "identifier on left side of :=")
		}
	}
	if n == 0 && p.mode&DeclarationErrors != 0 {
		p.error(list[0].Pos(), "no new variables on left side of :=")
	}
}

The unresolved object is a sentinel to mark identifiers that have been added to the list of unresolved identifiers. The sentinel is only used for verifying internal consistency.

var unresolved = new(ast.Object)

If x is an identifier, tryResolve attempts to resolve x by looking up the object it denotes. If no object is found and collectUnresolved is set, x is marked as unresolved and collected in the list of unresolved identifiers.

nothing to do if x is not an identifier or the blank identifier

	ident, _ := x.(*ast.Ident)
	if ident == nil {
		return
	}
	assert(ident.Obj == nil, "identifier already declared or resolved")
	if ident.Name == "_" {
		return

try to resolve the identifier

	for s := p.topScope; s != nil; s = s.Outer {
		if obj := s.Lookup(ident.Name); obj != nil {
			ident.Obj = obj
			return
		}

all local scopes are known, so any unresolved identifier must be found either in the file scope, package scope (perhaps in another file), or universe scope --- collect them so that they can be resolved later

	if collectUnresolved {
		ident.Obj = unresolved
		p.unresolved = append(p.unresolved, ident)
	}
}

func (p *parser) resolve(x ast.Expr) {
	p.tryResolve(x, true)
}

---------------------------------------------------------------------------- Parsing support


func (p *parser) printTrace(a ...interface{}) {
	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
	const n = len(dots)
	pos := p.file.Position(p.pos)
	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
	i := 2 * p.indent
	for i > n {
		fmt.Print(dots)
		i -= n

i <= n

	fmt.Print(dots[0:i])
	fmt.Println(a...)
}

func trace(p *parser, msg string) *parser {
	p.printTrace(msg, "(")
	p.indent++
	return p
}

Usage pattern: defer un(trace(p, "..."))

func un(p *parser) {
	p.indent--
	p.printTrace(")")
}

Advance to the next token.

Because of one-token look-ahead, print the previous token when tracing as it provides a more readable output. The very first token (!p.pos.IsValid()) is not initialized (it is token.ILLEGAL), so don't print it .

	if p.trace && p.pos.IsValid() {
		s := p.tok.String()
		switch {
		case p.tok.IsLiteral():
			p.printTrace(s, p.lit)
		case p.tok.IsOperator(), p.tok.IsKeyword():
			p.printTrace("\"" + s + "\"")
		default:
			p.printTrace(s)
		}
	}

	p.pos, p.tok, p.lit = p.scanner.Scan()
}

Consume a comment and return it and the line on which it ends.

-style comments may end on a different line than where they start. Scan the comment for '\n' chars and adjust endline accordingly.

	endline = p.file.Line(p.pos)

don't use range here - no need to decode Unicode code points

		for i := 0; i < len(p.lit); i++ {
			if p.lit[i] == '\n' {
				endline++
			}
		}
	}

	comment = &ast.Comment{Slash: p.pos, Text: p.lit}
	p.next0()

	return
}

Consume a group of adjacent comments, add it to the parser's comments list, and return it together with the line at which the last comment in the group ends. A non-comment token or n empty lines terminate a comment group.

func (p *parser) consumeCommentGroup(n int) (comments *ast.CommentGroup, endline int) {
	var list []*ast.Comment
	endline = p.file.Line(p.pos)
	for p.tok == token.COMMENT && p.file.Line(p.pos) <= endline+n {
		var comment *ast.Comment
		comment, endline = p.consumeComment()
		list = append(list, comment)
	}

add comment group to the comments list

	comments = &ast.CommentGroup{List: list}
	p.comments = append(p.comments, comments)

	return
}

Advance to the next non-comment token. In the process, collect any comment groups encountered, and remember the last lead and line comments. A lead comment is a comment group that starts and ends in a line without any other tokens and that is followed by a non-comment token on the line immediately after the comment group. A line comment is a comment group that follows a non-comment token on the same line, and that has no tokens after it on the line where it ends. Lead and line comments may be considered documentation that is stored in the AST.

func (p *parser) next() {
	p.leadComment = nil
	p.lineComment = nil
	prev := p.pos
	p.next0()

	if p.tok == token.COMMENT {
		var comment *ast.CommentGroup
		var endline int

The comment is on same line as the previous token; it cannot be a lead comment but may be a line comment.

			comment, endline = p.consumeCommentGroup(0)

The next token is on a different line, thus the last comment group is a line comment.

				p.lineComment = comment
			}
		}

consume successor comments, if any

		endline = -1
		for p.tok == token.COMMENT {
			comment, endline = p.consumeCommentGroup(1)
		}

The next token is following on the line immediately after the comment group, thus the last comment group is a lead comment.

			p.leadComment = comment
		}
	}
}

A bailout panic is raised to indicate early termination.

type bailout struct{}

func (p *parser) error(pos token.Pos, msg string) {
	epos := p.file.Position(pos)

If AllErrors is not set, discard errors reported on the same line as the last recorded error and stop parsing if there are more than 10 errors.

	if p.mode&AllErrors == 0 {
		n := len(p.errors)
		if n > 0 && p.errors[n-1].Pos.Line == epos.Line {
			return // discard - likely a spurious error
		}
		if n > 10 {
			panic(bailout{})
		}
	}

	p.errors.Add(epos, msg)
}

func (p *parser) errorExpected(pos token.Pos, msg string) {
	msg = "expected " + msg

the error happened at the current position; make the error message more specific

		switch {
		case p.tok == token.SEMICOLON && p.lit == "\n":
			msg += ", found newline"

print 123 rather than 'INT', etc.

			msg += ", found " + p.lit
		default:
			msg += ", found '" + p.tok.String() + "'"
		}
	}
	p.error(pos, msg)
}

func (p *parser) expect(tok token.Token) token.Pos {
	pos := p.pos
	if p.tok != tok {
		p.errorExpected(pos, "'"+tok.String()+"'")
	}
	p.next() // make progress
	return pos
}

expect2 is like expect, but it returns an invalid position if the expected token is not found.

func (p *parser) expect2(tok token.Token) (pos token.Pos) {
	if p.tok == tok {
		pos = p.pos
	} else {
		p.errorExpected(p.pos, "'"+tok.String()+"'")
	}
	p.next() // make progress
	return
}

expectClosing is like expect but provides a better error message for the common case of a missing comma before a newline.

func (p *parser) expectClosing(tok token.Token, context string) token.Pos {
	if p.tok != tok && p.tok == token.SEMICOLON && p.lit == "\n" {
		p.error(p.pos, "missing ',' before newline in "+context)
		p.next()
	}
	return p.expect(tok)
}

semicolon is optional before a closing ')' or '}'

	if p.tok != token.RPAREN && p.tok != token.RBRACE {
		switch p.tok {

permit a ',' instead of a ';' but complain

			p.errorExpected(p.pos, "';'")
			fallthrough
		case token.SEMICOLON:
			p.next()
		default:
			p.errorExpected(p.pos, "';'")
			p.advance(stmtStart)
		}
	}
}

func (p *parser) atComma(context string, follow token.Token) bool {
	if p.tok == token.COMMA {
		return true
	}
	if p.tok != follow {
		msg := "missing ','"
		if p.tok == token.SEMICOLON && p.lit == "\n" {
			msg += " before newline"
		}
		p.error(p.pos, msg+" in "+context)
		return true // "insert" comma and continue
	}
	return false
}

func assert(cond bool, msg string) {
	if !cond {
		panic("go/parser internal error: " + msg)
	}
}

advance consumes tokens until the current token p.tok is in the 'to' set, or token.EOF. For error recovery.

func (p *parser) advance(to map[token.Token]bool) {
	for ; p.tok != token.EOF; p.next() {

Return only if parser made some progress since last sync or if it has not reached 10 advance calls without progress. Otherwise consume at least one token to avoid an endless parser loop (it is possible that both parseOperand and parseStmt call advance and correctly do not advance, thus the need for the invocation limit p.syncCnt).

			if p.pos == p.syncPos && p.syncCnt < 10 {
				p.syncCnt++
				return
			}
			if p.pos > p.syncPos {
				p.syncPos = p.pos
				p.syncCnt = 0
				return

Reaching here indicates a parser bug, likely an incorrect token list in this function, but it only leads to skipping of possibly correct code if a previous error is present, and thus is preferred over a non-terminating parse.

		}
	}
}

var stmtStart = map[token.Token]bool{
	token.BREAK:       true,
	token.CONST:       true,
	token.CONTINUE:    true,
	token.DEFER:       true,
	token.FALLTHROUGH: true,
	token.FOR:         true,
	token.GO:          true,
	token.GOTO:        true,
	token.IF:          true,
	token.RETURN:      true,
	token.SELECT:      true,
	token.SWITCH:      true,
	token.TYPE:        true,
	token.VAR:         true,
}

var declStart = map[token.Token]bool{
	token.CONST: true,
	token.TYPE:  true,
	token.VAR:   true,
}

var exprEnd = map[token.Token]bool{
	token.COMMA:     true,
	token.COLON:     true,
	token.SEMICOLON: true,
	token.RPAREN:    true,
	token.RBRACK:    true,
	token.RBRACE:    true,
}

safePos returns a valid file position for a given position: If pos is valid to begin with, safePos returns pos. If pos is out-of-range, safePos returns the EOF position. This is hack to work around "artificial" end positions in the AST which are computed by adding 1 to (presumably valid) token positions. If the token positions are invalid due to parse errors, the resulting end position may be past the file's EOF position, which would lead to panics if used later on.

func (p *parser) safePos(pos token.Pos) (res token.Pos) {
	defer func() {
		if recover() != nil {
			res = token.Pos(p.file.Base() + p.file.Size()) // EOF position
		}
	}()
	_ = p.file.Offset(pos) // trigger a panic if position is out-of-range
	return pos
}

---------------------------------------------------------------------------- Identifiers


func (p *parser) parseIdent() *ast.Ident {
	pos := p.pos
	name := "_"
	if p.tok == token.IDENT {
		name = p.lit
		p.next()
	} else {
		p.expect(token.IDENT) // use expect() error handling
	}
	return &ast.Ident{NamePos: pos, Name: name}
}

func (p *parser) parseIdentList() (list []*ast.Ident) {
	if p.trace {
		defer un(trace(p, "IdentList"))
	}

	list = append(list, p.parseIdent())
	for p.tok == token.COMMA {
		p.next()
		list = append(list, p.parseIdent())
	}

	return
}

---------------------------------------------------------------------------- Common productions

If lhs is set, result list elements which are identifiers are not resolved.

func (p *parser) parseExprList(lhs bool) (list []ast.Expr) {
	if p.trace {
		defer un(trace(p, "ExpressionList"))
	}

	list = append(list, p.checkExpr(p.parseExpr(lhs)))
	for p.tok == token.COMMA {
		p.next()
		list = append(list, p.checkExpr(p.parseExpr(lhs)))
	}

	return
}

func (p *parser) parseLhsList() []ast.Expr {
	old := p.inRhs
	p.inRhs = false
	list := p.parseExprList(true)
	switch p.tok {

lhs of a short variable declaration but doesn't enter scope until later: caller must call p.shortVarDecl(p.makeIdentList(list)) at appropriate time.

lhs of a label declaration or a communication clause of a select statement (parseLhsList is not called when parsing the case clause of a switch statement): - labels are declared by the caller of parseLhsList - for communication clauses, if there is a stand-alone identifier followed by a colon, we have a syntax error; there is no need to resolve the identifier in that case

identifiers must be declared elsewhere

		for _, x := range list {
			p.resolve(x)
		}
	}
	p.inRhs = old
	return list
}

func (p *parser) parseRhsList() []ast.Expr {
	old := p.inRhs
	p.inRhs = true
	list := p.parseExprList(false)
	p.inRhs = old
	return list
}

---------------------------------------------------------------------------- Types


func (p *parser) parseType() ast.Expr {
	if p.trace {
		defer un(trace(p, "Type"))
	}

	typ := p.tryType()

	if typ == nil {
		pos := p.pos
		p.errorExpected(pos, "type")
		p.advance(exprEnd)
		return &ast.BadExpr{From: pos, To: p.pos}
	}

	return typ
}

If the result is an identifier, it is not resolved.

func (p *parser) parseTypeName() ast.Expr {
	if p.trace {
		defer un(trace(p, "TypeName"))
	}

don't resolve ident yet - it may be a parameter or field name

ident is a package name

		p.next()
		p.resolve(ident)
		sel := p.parseIdent()
		return &ast.SelectorExpr{X: ident, Sel: sel}
	}

	return ident
}

func (p *parser) parseArrayType() ast.Expr {
	if p.trace {
		defer un(trace(p, "ArrayType"))
	}

	lbrack := p.expect(token.LBRACK)
	p.exprLev++

always permit ellipsis for more fault-tolerant parsing

	if p.tok == token.ELLIPSIS {
		len = &ast.Ellipsis{Ellipsis: p.pos}
		p.next()
	} else if p.tok != token.RBRACK {
		len = p.parseRhs()
	}
	p.exprLev--
	p.expect(token.RBRACK)
	elt := p.parseType()

	return &ast.ArrayType{Lbrack: lbrack, Len: len, Elt: elt}
}

func (p *parser) makeIdentList(list []ast.Expr) []*ast.Ident {
	idents := make([]*ast.Ident, len(list))
	for i, x := range list {
		ident, isIdent := x.(*ast.Ident)
		if !isIdent {

only report error if it's a new one

				p.errorExpected(x.Pos(), "identifier")
			}
			ident = &ast.Ident{NamePos: x.Pos(), Name: "_"}
		}
		idents[i] = ident
	}
	return idents
}

func (p *parser) parseFieldDecl(scope *ast.Scope) *ast.Field {
	if p.trace {
		defer un(trace(p, "FieldDecl"))
	}

	doc := p.leadComment

1st FieldDecl A type name used as an anonymous field looks like a field identifier.

	var list []ast.Expr
	for {
		list = append(list, p.parseVarType(false))
		if p.tok != token.COMMA {
			break
		}
		p.next()
	}

	typ := p.tryVarType(false)

analyze case

	var idents []*ast.Ident

IdentifierList Type

		idents = p.makeIdentList(list)

["*"] TypeName (AnonymousField)

		typ = list[0] // we always have at least one element
		if n := len(list); n > 1 {
			p.errorExpected(p.pos, "type")
			typ = &ast.BadExpr{From: p.pos, To: p.pos}
		} else if !isTypeName(deref(typ)) {
			p.errorExpected(typ.Pos(), "anonymous field")
			typ = &ast.BadExpr{From: typ.Pos(), To: p.safePos(typ.End())}
		}
	}

Tag

	var tag *ast.BasicLit
	if p.tok == token.STRING {
		tag = &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
		p.next()
	}

	p.expectSemi() // call before accessing p.linecomment

	field := &ast.Field{Doc: doc, Names: idents, Type: typ, Tag: tag, Comment: p.lineComment}
	p.declare(field, nil, scope, ast.Var, idents...)
	p.resolve(typ)

	return field
}

func (p *parser) parseStructType() *ast.StructType {
	if p.trace {
		defer un(trace(p, "StructType"))
	}

	pos := p.expect(token.STRUCT)
	lbrace := p.expect(token.LBRACE)
	scope := ast.NewScope(nil) // struct scope
	var list []*ast.Field

a field declaration cannot start with a '(' but we accept it here for more robust parsing and better error messages (parseFieldDecl will check and complain if necessary)

		list = append(list, p.parseFieldDecl(scope))
	}
	rbrace := p.expect(token.RBRACE)

	return &ast.StructType{
		Struct: pos,
		Fields: &ast.FieldList{
			Opening: lbrace,
			List:    list,
			Closing: rbrace,
		},
	}
}

func (p *parser) parsePointerType() *ast.StarExpr {
	if p.trace {
		defer un(trace(p, "PointerType"))
	}

	star := p.expect(token.MUL)
	base := p.parseType()

	return &ast.StarExpr{Star: star, X: base}
}

If the result is an identifier, it is not resolved.

func (p *parser) tryVarType(isParam bool) ast.Expr {
	if isParam && p.tok == token.ELLIPSIS {
		pos := p.pos
		p.next()
		typ := p.tryIdentOrType() // don't use parseType so we can provide better error message
		if typ != nil {
			p.resolve(typ)
		} else {
			p.error(pos, "'...' parameter is missing type")
			typ = &ast.BadExpr{From: pos, To: p.pos}
		}
		return &ast.Ellipsis{Ellipsis: pos, Elt: typ}
	}
	return p.tryIdentOrType()
}

If the result is an identifier, it is not resolved.

func (p *parser) parseVarType(isParam bool) ast.Expr {
	typ := p.tryVarType(isParam)
	if typ == nil {
		pos := p.pos
		p.errorExpected(pos, "type")
		p.next() // make progress
		typ = &ast.BadExpr{From: pos, To: p.pos}
	}
	return typ
}

func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params []*ast.Field) {
	if p.trace {
		defer un(trace(p, "ParameterList"))
	}

1st ParameterDecl A list of identifiers looks like a list of type names.

	var list []ast.Expr
	for {
		list = append(list, p.parseVarType(ellipsisOk))
		if p.tok != token.COMMA {
			break
		}
		p.next()
		if p.tok == token.RPAREN {
			break
		}
	}

analyze case

IdentifierList Type

		idents := p.makeIdentList(list)
		field := &ast.Field{Names: idents, Type: typ}

Go spec: The scope of an identifier denoting a function parameter or result variable is the function body.

		p.declare(field, nil, scope, ast.Var, idents...)
		p.resolve(typ)
		if !p.atComma("parameter list", token.RPAREN) {
			return
		}
		p.next()
		for p.tok != token.RPAREN && p.tok != token.EOF {
			idents := p.parseIdentList()
			typ := p.parseVarType(ellipsisOk)
			field := &ast.Field{Names: idents, Type: typ}

Go spec: The scope of an identifier denoting a function parameter or result variable is the function body.

			p.declare(field, nil, scope, ast.Var, idents...)
			p.resolve(typ)
			if !p.atComma("parameter list", token.RPAREN) {
				break
			}
			p.next()
		}
		return
	}

Type { "," Type } (anonymous parameters)

	params = make([]*ast.Field, len(list))
	for i, typ := range list {
		p.resolve(typ)
		params[i] = &ast.Field{Type: typ}
	}
	return
}

func (p *parser) parseParameters(scope *ast.Scope, ellipsisOk bool) *ast.FieldList {
	if p.trace {
		defer un(trace(p, "Parameters"))
	}

	var params []*ast.Field
	lparen := p.expect(token.LPAREN)
	if p.tok != token.RPAREN {
		params = p.parseParameterList(scope, ellipsisOk)
	}
	rparen := p.expect(token.RPAREN)

	return &ast.FieldList{Opening: lparen, List: params, Closing: rparen}
}

func (p *parser) parseResult(scope *ast.Scope) *ast.FieldList {
	if p.trace {
		defer un(trace(p, "Result"))
	}

	if p.tok == token.LPAREN {
		return p.parseParameters(scope, false)
	}

	typ := p.tryType()
	if typ != nil {
		list := make([]*ast.Field, 1)
		list[0] = &ast.Field{Type: typ}
		return &ast.FieldList{List: list}
	}

	return nil
}

func (p *parser) parseSignature(scope *ast.Scope) (params, results *ast.FieldList) {
	if p.trace {
		defer un(trace(p, "Signature"))
	}

	params = p.parseParameters(scope, true)
	results = p.parseResult(scope)

	return
}

func (p *parser) parseFuncType() (*ast.FuncType, *ast.Scope) {
	if p.trace {
		defer un(trace(p, "FuncType"))
	}

	pos := p.expect(token.FUNC)
	scope := ast.NewScope(p.topScope) // function scope
	params, results := p.parseSignature(scope)

	return &ast.FuncType{Func: pos, Params: params, Results: results}, scope
}

func (p *parser) parseMethodSpec(scope *ast.Scope) *ast.Field {
	if p.trace {
		defer un(trace(p, "MethodSpec"))
	}

	doc := p.leadComment
	var idents []*ast.Ident
	var typ ast.Expr
	x := p.parseTypeName()

method

		idents = []*ast.Ident{ident}
		scope := ast.NewScope(nil) // method scope
		params, results := p.parseSignature(scope)
		typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}

embedded interface

		typ = x
		p.resolve(typ)
	}
	p.expectSemi() // call before accessing p.linecomment

	spec := &ast.Field{Doc: doc, Names: idents, Type: typ, Comment: p.lineComment}
	p.declare(spec, nil, scope, ast.Fun, idents...)

	return spec
}

func (p *parser) parseInterfaceType() *ast.InterfaceType {
	if p.trace {
		defer un(trace(p, "InterfaceType"))
	}

	pos := p.expect(token.INTERFACE)
	lbrace := p.expect(token.LBRACE)
	scope := ast.NewScope(nil) // interface scope
	var list []*ast.Field
	for p.tok == token.IDENT {
		list = append(list, p.parseMethodSpec(scope))
	}
	rbrace := p.expect(token.RBRACE)

	return &ast.InterfaceType{
		Interface: pos,
		Methods: &ast.FieldList{
			Opening: lbrace,
			List:    list,
			Closing: rbrace,
		},
	}
}

func (p *parser) parseMapType() *ast.MapType {
	if p.trace {
		defer un(trace(p, "MapType"))
	}

	pos := p.expect(token.MAP)
	p.expect(token.LBRACK)
	key := p.parseType()
	p.expect(token.RBRACK)
	value := p.parseType()

	return &ast.MapType{Map: pos, Key: key, Value: value}
}

func (p *parser) parseChanType() *ast.ChanType {
	if p.trace {
		defer un(trace(p, "ChanType"))
	}

	pos := p.pos
	dir := ast.SEND | ast.RECV
	var arrow token.Pos
	if p.tok == token.CHAN {
		p.next()
		if p.tok == token.ARROW {
			arrow = p.pos
			p.next()
			dir = ast.SEND
		}
	} else {
		arrow = p.expect(token.ARROW)
		p.expect(token.CHAN)
		dir = ast.RECV
	}
	value := p.parseType()

	return &ast.ChanType{Begin: pos, Arrow: arrow, Dir: dir, Value: value}
}

If the result is an identifier, it is not resolved.

func (p *parser) tryIdentOrType() ast.Expr {
	switch p.tok {
	case token.IDENT:
		return p.parseTypeName()
	case token.LBRACK:
		return p.parseArrayType()
	case token.STRUCT:
		return p.parseStructType()
	case token.MUL:
		return p.parsePointerType()
	case token.FUNC:
		typ, _ := p.parseFuncType()
		return typ
	case token.INTERFACE:
		return p.parseInterfaceType()
	case token.MAP:
		return p.parseMapType()
	case token.CHAN, token.ARROW:
		return p.parseChanType()
	case token.LPAREN:
		lparen := p.pos
		p.next()
		typ := p.parseType()
		rparen := p.expect(token.RPAREN)
		return &ast.ParenExpr{Lparen: lparen, X: typ, Rparen: rparen}
	}

no type found

	return nil
}

func (p *parser) tryType() ast.Expr {
	typ := p.tryIdentOrType()
	if typ != nil {
		p.resolve(typ)
	}
	return typ
}

---------------------------------------------------------------------------- Blocks


func (p *parser) parseStmtList() (list []ast.Stmt) {
	if p.trace {
		defer un(trace(p, "StatementList"))
	}

	for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
		list = append(list, p.parseStmt())
	}

	return
}

func (p *parser) parseBody(scope *ast.Scope) *ast.BlockStmt {
	if p.trace {
		defer un(trace(p, "Body"))
	}

	lbrace := p.expect(token.LBRACE)
	p.topScope = scope // open function scope
	p.openLabelScope()
	list := p.parseStmtList()
	p.closeLabelScope()
	p.closeScope()
	rbrace := p.expect2(token.RBRACE)

	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
}

func (p *parser) parseBlockStmt() *ast.BlockStmt {
	if p.trace {
		defer un(trace(p, "BlockStmt"))
	}

	lbrace := p.expect(token.LBRACE)
	p.openScope()
	list := p.parseStmtList()
	p.closeScope()
	rbrace := p.expect2(token.RBRACE)

	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
}

---------------------------------------------------------------------------- Expressions


func (p *parser) parseFuncTypeOrLit() ast.Expr {
	if p.trace {
		defer un(trace(p, "FuncTypeOrLit"))
	}

	typ, scope := p.parseFuncType()

function type only

		return typ
	}

	p.exprLev++
	body := p.parseBody(scope)
	p.exprLev--

	return &ast.FuncLit{Type: typ, Body: body}
}

parseOperand may return an expression or a raw type (incl. array types of the form [...]T. Callers must verify the result. If lhs is set and the result is an identifier, it is not resolved.

func (p *parser) parseOperand(lhs bool) ast.Expr {
	if p.trace {
		defer un(trace(p, "Operand"))
	}

	switch p.tok {
	case token.IDENT:
		x := p.parseIdent()
		if !lhs {
			p.resolve(x)
		}
		return x

	case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
		x := &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
		p.next()
		return x

	case token.LPAREN:
		lparen := p.pos
		p.next()
		p.exprLev++
		x := p.parseRhsOrType() // types may be parenthesized: (some type)
		p.exprLev--
		rparen := p.expect(token.RPAREN)
		return &ast.ParenExpr{Lparen: lparen, X: x, Rparen: rparen}

	case token.FUNC:
		return p.parseFuncTypeOrLit()
	}

could be type for composite literal or conversion

		_, isIdent := typ.(*ast.Ident)
		assert(!isIdent, "type cannot be identifier")
		return typ
	}

we have an error

	pos := p.pos
	p.errorExpected(pos, "operand")
	p.advance(stmtStart)
	return &ast.BadExpr{From: pos, To: p.pos}
}

func (p *parser) parseSelector(x ast.Expr) ast.Expr {
	if p.trace {
		defer un(trace(p, "Selector"))
	}

	sel := p.parseIdent()

	return &ast.SelectorExpr{X: x, Sel: sel}
}

func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
	if p.trace {
		defer un(trace(p, "TypeAssertion"))
	}

	lparen := p.expect(token.LPAREN)
	var typ ast.Expr

type switch: typ == nil

		p.next()
	} else {
		typ = p.parseType()
	}
	rparen := p.expect(token.RPAREN)

	return &ast.TypeAssertExpr{X: x, Type: typ, Lparen: lparen, Rparen: rparen}
}

func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
	if p.trace {
		defer un(trace(p, "IndexOrSlice"))
	}

	const N = 3 // change the 3 to 2 to disable 3-index slices
	lbrack := p.expect(token.LBRACK)
	p.exprLev++
	var index [N]ast.Expr
	var colons [N - 1]token.Pos
	if p.tok != token.COLON {
		index[0] = p.parseRhs()
	}
	ncolons := 0
	for p.tok == token.COLON && ncolons < len(colons) {
		colons[ncolons] = p.pos
		ncolons++
		p.next()
		if p.tok != token.COLON && p.tok != token.RBRACK && p.tok != token.EOF {
			index[ncolons] = p.parseRhs()
		}
	}
	p.exprLev--
	rbrack := p.expect(token.RBRACK)

slice expression

		slice3 := false
		if ncolons == 2 {

Check presence of 2nd and 3rd index here rather than during type-checking to prevent erroneous programs from passing through gofmt (was issue 7305).

			if index[1] == nil {
				p.error(colons[0], "2nd index required in 3-index slice")
				index[1] = &ast.BadExpr{From: colons[0] + 1, To: colons[1]}
			}
			if index[2] == nil {
				p.error(colons[1], "3rd index required in 3-index slice")
				index[2] = &ast.BadExpr{From: colons[1] + 1, To: rbrack}
			}
		}
		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: slice3, Rbrack: rbrack}
	}

	return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}
}

func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
	if p.trace {
		defer un(trace(p, "CallOrConversion"))
	}

	lparen := p.expect(token.LPAREN)
	p.exprLev++
	var list []ast.Expr
	var ellipsis token.Pos
	for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
		list = append(list, p.parseRhsOrType()) // builtins may expect a type: make(some type, ...)
		if p.tok == token.ELLIPSIS {
			ellipsis = p.pos
			p.next()
		}
		if !p.atComma("argument list", token.RPAREN) {
			break
		}
		p.next()
	}
	p.exprLev--
	rparen := p.expectClosing(token.RPAREN, "argument list")

	return &ast.CallExpr{Fun: fun, Lparen: lparen, Args: list, Ellipsis: ellipsis, Rparen: rparen}
}

func (p *parser) parseValue(keyOk bool) ast.Expr {
	if p.trace {
		defer un(trace(p, "Element"))
	}

	if p.tok == token.LBRACE {
		return p.parseLiteralValue(nil)
	}

Because the parser doesn't know the composite literal type, it cannot know if a key that's an identifier is a struct field name or a name denoting a value. The former is not resolved by the parser or the resolver. Instead, _try_ to resolve such a key if possible. If it resolves, it a) has correctly resolved, or b) incorrectly resolved because the key is a struct field with a name matching another identifier. In the former case we are done, and in the latter case we don't care because the type checker will do a separate field lookup. If the key does not resolve, it a) must be defined at the top level in another file of the same package, the universe scope, or be undeclared; or b) it is a struct field. In the former case, the type checker can do a top-level lookup, and in the latter case it will do a separate field lookup.

	x := p.checkExpr(p.parseExpr(keyOk))
	if keyOk {

Try to resolve the key but don't collect it as unresolved identifier if it fails so that we don't get (possibly false) errors about undeclared names.

			p.tryResolve(x, false)

not a key

			p.resolve(x)
		}
	}

	return x
}

func (p *parser) parseElement() ast.Expr {
	if p.trace {
		defer un(trace(p, "Element"))
	}

	x := p.parseValue(true)
	if p.tok == token.COLON {
		colon := p.pos
		p.next()
		x = &ast.KeyValueExpr{Key: x, Colon: colon, Value: p.parseValue(false)}
	}

	return x
}

func (p *parser) parseElementList() (list []ast.Expr) {
	if p.trace {
		defer un(trace(p, "ElementList"))
	}

	for p.tok != token.RBRACE && p.tok != token.EOF {
		list = append(list, p.parseElement())
		if !p.atComma("composite literal", token.RBRACE) {
			break
		}
		p.next()
	}

	return
}

func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
	if p.trace {
		defer un(trace(p, "LiteralValue"))
	}

	lbrace := p.expect(token.LBRACE)
	var elts []ast.Expr
	p.exprLev++
	if p.tok != token.RBRACE {
		elts = p.parseElementList()
	}
	p.exprLev--
	rbrace := p.expectClosing(token.RBRACE, "composite literal")
	return &ast.CompositeLit{Type: typ, Lbrace: lbrace, Elts: elts, Rbrace: rbrace}
}

checkExpr checks that x is an expression (and not a type).

func (p *parser) checkExpr(x ast.Expr) ast.Expr {
	switch unparen(x).(type) {
	case *ast.BadExpr:
	case *ast.Ident:
	case *ast.BasicLit:
	case *ast.FuncLit:
	case *ast.CompositeLit:
	case *ast.ParenExpr:
		panic("unreachable")
	case *ast.SelectorExpr:
	case *ast.IndexExpr:
	case *ast.SliceExpr:

If t.Type == nil we have a type assertion of the form y.(type), which is only allowed in type switch expressions. It's hard to exclude those but for the case where we are in a type switch. Instead be lenient and test this in the type checker.

	case *ast.CallExpr:
	case *ast.StarExpr:
	case *ast.UnaryExpr:
	case *ast.BinaryExpr:

all other nodes are not proper expressions

		p.errorExpected(x.Pos(), "expression")
		x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
	}
	return x
}

isTypeName reports whether x is a (qualified) TypeName.

func isTypeName(x ast.Expr) bool {
	switch t := x.(type) {
	case *ast.BadExpr:
	case *ast.Ident:
	case *ast.SelectorExpr:
		_, isIdent := t.X.(*ast.Ident)
		return isIdent
	default:
		return false // all other nodes are not type names
	}
	return true
}

isLiteralType reports whether x is a legal composite literal type.

func isLiteralType(x ast.Expr) bool {
	switch t := x.(type) {
	case *ast.BadExpr:
	case *ast.Ident:
	case *ast.SelectorExpr:
		_, isIdent := t.X.(*ast.Ident)
		return isIdent
	case *ast.ArrayType:
	case *ast.StructType:
	case *ast.MapType:
	default:
		return false // all other nodes are not legal composite literal types
	}
	return true
}

If x is of the form *T, deref returns T, otherwise it returns x.

func deref(x ast.Expr) ast.Expr {
	if p, isPtr := x.(*ast.StarExpr); isPtr {
		x = p.X
	}
	return x
}

If x is of the form (T), unparen returns unparen(T), otherwise it returns x.

func unparen(x ast.Expr) ast.Expr {
	if p, isParen := x.(*ast.ParenExpr); isParen {
		x = unparen(p.X)
	}
	return x
}

checkExprOrType checks that x is an expression or a type (and not a raw type such as [...]T).

func (p *parser) checkExprOrType(x ast.Expr) ast.Expr {
	switch t := unparen(x).(type) {
	case *ast.ParenExpr:
		panic("unreachable")
	case *ast.ArrayType:
		if len, isEllipsis := t.Len.(*ast.Ellipsis); isEllipsis {
			p.error(len.Pos(), "expected array length, found '...'")
			x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
		}
	}

all other nodes are expressions or types

	return x
}

If lhs is set and the result is an identifier, it is not resolved.

func (p *parser) parsePrimaryExpr(lhs bool) ast.Expr {
	if p.trace {
		defer un(trace(p, "PrimaryExpr"))
	}

	x := p.parseOperand(lhs)
L:
	for {
		switch p.tok {
		case token.PERIOD:
			p.next()
			if lhs {
				p.resolve(x)
			}
			switch p.tok {
			case token.IDENT:
				x = p.parseSelector(p.checkExprOrType(x))
			case token.LPAREN:
				x = p.parseTypeAssertion(p.checkExpr(x))
			default:
				pos := p.pos
				p.errorExpected(pos, "selector or type assertion")
				p.next() // make progress
				sel := &ast.Ident{NamePos: pos, Name: "_"}
				x = &ast.SelectorExpr{X: x, Sel: sel}
			}
		case token.LBRACK:
			if lhs {
				p.resolve(x)
			}
			x = p.parseIndexOrSlice(p.checkExpr(x))
		case token.LPAREN:
			if lhs {
				p.resolve(x)
			}
			x = p.parseCallOrConversion(p.checkExprOrType(x))
		case token.LBRACE:
			if isLiteralType(x) && (p.exprLev >= 0 || !isTypeName(x)) {
				if lhs {
					p.resolve(x)
				}
				x = p.parseLiteralValue(x)
			} else {
				break L
			}
		default:
			break L
		}
		lhs = false // no need to try to resolve again
	}

	return x
}

If lhs is set and the result is an identifier, it is not resolved.

func (p *parser) parseUnaryExpr(lhs bool) ast.Expr {
	if p.trace {
		defer un(trace(p, "UnaryExpr"))
	}

	switch p.tok {
	case token.ADD, token.SUB, token.NOT, token.XOR, token.AND:
		pos, op := p.pos, p.tok
		p.next()
		x := p.parseUnaryExpr(false)
		return &ast.UnaryExpr{OpPos: pos, Op: op, X: p.checkExpr(x)}

channel type or receive expression

		arrow := p.pos
		p.next()

If the next token is token.CHAN we still don't know if it is a channel type or a receive operation - we only know once we have found the end of the unary expression. There are two cases: <- type => (<-type) must be channel type <- expr => <-(expr) is a receive from an expression In the first case, the arrow must be re-associated with the channel type parsed already: <- (chan type) => (<-chan type) <- (chan<- type) => (<-chan (<-type))


		x := p.parseUnaryExpr(false)

determine which case we have

(<-type)

re-associate position info and <-

			dir := ast.SEND
			for ok && dir == ast.SEND {

error: (<-type) is (<-(<-chan T))

					p.errorExpected(typ.Arrow, "'chan'")
				}
				arrow, typ.Begin, typ.Arrow = typ.Arrow, arrow, arrow
				dir, typ.Dir = typ.Dir, ast.RECV
				typ, ok = typ.Value.(*ast.ChanType)
			}
			if dir == ast.SEND {
				p.errorExpected(arrow, "channel type")
			}

			return x
		}

<-(expr)

		return &ast.UnaryExpr{OpPos: arrow, Op: token.ARROW, X: p.checkExpr(x)}

pointer type or unary "*" expression

		pos := p.pos
		p.next()
		x := p.parseUnaryExpr(false)
		return &ast.StarExpr{Star: pos, X: p.checkExprOrType(x)}
	}

	return p.parsePrimaryExpr(lhs)
}

func (p *parser) tokPrec() (token.Token, int) {
	tok := p.tok
	if p.inRhs && tok == token.ASSIGN {
		tok = token.EQL
	}
	return tok, tok.Precedence()
}

If lhs is set and the result is an identifier, it is not resolved.

func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
	if p.trace {
		defer un(trace(p, "BinaryExpr"))
	}

	x := p.parseUnaryExpr(lhs)
	for {
		op, oprec := p.tokPrec()
		if oprec < prec1 {
			return x
		}
		pos := p.expect(op)
		if lhs {
			p.resolve(x)
			lhs = false
		}
		y := p.parseBinaryExpr(false, oprec+1)
		x = &ast.BinaryExpr{X: p.checkExpr(x), OpPos: pos, Op: op, Y: p.checkExpr(y)}
	}
}

If lhs is set and the result is an identifier, it is not resolved. The result may be a type or even a raw type ([...]int). Callers must check the result (using checkExpr or checkExprOrType), depending on context.

func (p *parser) parseExpr(lhs bool) ast.Expr {
	if p.trace {
		defer un(trace(p, "Expression"))
	}

	return p.parseBinaryExpr(lhs, token.LowestPrec+1)
}

func (p *parser) parseRhs() ast.Expr {
	old := p.inRhs
	p.inRhs = true
	x := p.checkExpr(p.parseExpr(false))
	p.inRhs = old
	return x
}

func (p *parser) parseRhsOrType() ast.Expr {
	old := p.inRhs
	p.inRhs = true
	x := p.checkExprOrType(p.parseExpr(false))
	p.inRhs = old
	return x
}

---------------------------------------------------------------------------- Statements

Parsing modes for parseSimpleStmt.

const (
	basic = iota
	labelOk
	rangeOk
)

parseSimpleStmt returns true as 2nd result if it parsed the assignment of a range clause (with mode == rangeOk). The returned statement is an assignment with a right-hand side that is a single unary expression of the form "range x". No guarantees are given for the left-hand side.

func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
	if p.trace {
		defer un(trace(p, "SimpleStmt"))
	}

	x := p.parseLhsList()

	switch p.tok {
	case
		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
		token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,

assignment statement, possibly part of a range clause

		pos, tok := p.pos, p.tok
		p.next()
		var y []ast.Expr
		isRange := false
		if mode == rangeOk && p.tok == token.RANGE && (tok == token.DEFINE || tok == token.ASSIGN) {
			pos := p.pos
			p.next()
			y = []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
			isRange = true
		} else {
			y = p.parseRhsList()
		}
		as := &ast.AssignStmt{Lhs: x, TokPos: pos, Tok: tok, Rhs: y}
		if tok == token.DEFINE {
			p.shortVarDecl(as, x)
		}
		return as, isRange
	}

	if len(x) > 1 {

continue with first expression

	}

	switch p.tok {

labeled statement

		colon := p.pos
		p.next()

Go spec: The scope of a label is the body of the function in which it is declared and excludes the body of any nested function.

			stmt := &ast.LabeledStmt{Label: label, Colon: colon, Stmt: p.parseStmt()}
			p.declare(stmt, nil, p.labelScope, ast.Lbl, label)
			return stmt, false

The label declaration typically starts at x[0].Pos(), but the label declaration may be erroneous due to a token after that position (and before the ':'). If SpuriousErrors is not set, the (only) error reported for the line is the illegal label error instead of the token before the ':' that caused the problem. Thus, use the (latest) colon position for error reporting.

		p.error(colon, "illegal label declaration")
		return &ast.BadStmt{From: x[0].Pos(), To: colon + 1}, false

send statement

		arrow := p.pos
		p.next()
		y := p.parseRhs()
		return &ast.SendStmt{Chan: x[0], Arrow: arrow, Value: y}, false

increment or decrement

		s := &ast.IncDecStmt{X: x[0], TokPos: p.pos, Tok: p.tok}
		p.next()
		return s, false
	}

expression

	return &ast.ExprStmt{X: x[0]}, false
}

func (p *parser) parseCallExpr(callType string) *ast.CallExpr {
	x := p.parseRhsOrType() // could be a conversion: (some type)(x)
	if call, isCall := x.(*ast.CallExpr); isCall {
		return call
	}

only report error if it's a new one

		p.error(p.safePos(x.End()), fmt.Sprintf("function must be invoked in %s statement", callType))
	}
	return nil
}

func (p *parser) parseGoStmt() ast.Stmt {
	if p.trace {
		defer un(trace(p, "GoStmt"))
	}

	pos := p.expect(token.GO)
	call := p.parseCallExpr("go")
	p.expectSemi()
	if call == nil {
		return &ast.BadStmt{From: pos, To: pos + 2} // len("go")
	}

	return &ast.GoStmt{Go: pos, Call: call}
}

func (p *parser) parseDeferStmt() ast.Stmt {
	if p.trace {
		defer un(trace(p, "DeferStmt"))
	}

	pos := p.expect(token.DEFER)
	call := p.parseCallExpr("defer")
	p.expectSemi()
	if call == nil {
		return &ast.BadStmt{From: pos, To: pos + 5} // len("defer")
	}

	return &ast.DeferStmt{Defer: pos, Call: call}
}

func (p *parser) parseReturnStmt() *ast.ReturnStmt {
	if p.trace {
		defer un(trace(p, "ReturnStmt"))
	}

	pos := p.pos
	p.expect(token.RETURN)
	var x []ast.Expr
	if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
		x = p.parseRhsList()
	}
	p.expectSemi()

	return &ast.ReturnStmt{Return: pos, Results: x}
}

func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
	if p.trace {
		defer un(trace(p, "BranchStmt"))
	}

	pos := p.expect(tok)
	var label *ast.Ident
	if tok != token.FALLTHROUGH && p.tok == token.IDENT {

add to list of unresolved targets

		n := len(p.targetStack) - 1
		p.targetStack[n] = append(p.targetStack[n], label)
	}
	p.expectSemi()

	return &ast.BranchStmt{TokPos: pos, Tok: tok, Label: label}
}

func (p *parser) makeExpr(s ast.Stmt, want string) ast.Expr {
	if s == nil {
		return nil
	}
	if es, isExpr := s.(*ast.ExprStmt); isExpr {
		return p.checkExpr(es.X)
	}
	found := "simple statement"
	if _, isAss := s.(*ast.AssignStmt); isAss {
		found = "assignment"
	}
	p.error(s.Pos(), fmt.Sprintf("expected %s, found %s (missing parentheses around composite literal?)", want, found))
	return &ast.BadExpr{From: s.Pos(), To: p.safePos(s.End())}
}

parseIfHeader is an adjusted version of parser.header in cmd/compile/internal/syntax/parser.go, which has been tuned for better error handling.

func (p *parser) parseIfHeader() (init ast.Stmt, cond ast.Expr) {
	if p.tok == token.LBRACE {
		p.error(p.pos, "missing condition in if statement")
		cond = &ast.BadExpr{From: p.pos, To: p.pos}
		return

p.tok != token.LBRACE


	outer := p.exprLev
	p.exprLev = -1

accept potential variable declaration but complain

		if p.tok == token.VAR {
			p.next()
			p.error(p.pos, fmt.Sprintf("var declaration not allowed in 'IF' initializer"))
		}
		init, _ = p.parseSimpleStmt(basic)
	}

	var condStmt ast.Stmt
	var semi struct {
		pos token.Pos
		lit string // ";" or "\n"; valid if pos.IsValid()
	}
	if p.tok != token.LBRACE {
		if p.tok == token.SEMICOLON {
			semi.pos = p.pos
			semi.lit = p.lit
			p.next()
		} else {
			p.expect(token.SEMICOLON)
		}
		if p.tok != token.LBRACE {
			condStmt, _ = p.parseSimpleStmt(basic)
		}
	} else {
		condStmt = init
		init = nil
	}

	if condStmt != nil {
		cond = p.makeExpr(condStmt, "boolean expression")
	} else if semi.pos.IsValid() {
		if semi.lit == "\n" {
			p.error(semi.pos, "unexpected newline, expecting { after if clause")
		} else {
			p.error(semi.pos, "missing condition in if statement")
		}
	}

make sure we have a valid AST

	if cond == nil {
		cond = &ast.BadExpr{From: p.pos, To: p.pos}
	}

	p.exprLev = outer
	return
}

func (p *parser) parseIfStmt() *ast.IfStmt {
	if p.trace {
		defer un(trace(p, "IfStmt"))
	}

	pos := p.expect(token.IF)
	p.openScope()
	defer p.closeScope()

	init, cond := p.parseIfHeader()
	body := p.parseBlockStmt()

	var else_ ast.Stmt
	if p.tok == token.ELSE {
		p.next()
		switch p.tok {
		case token.IF:
			else_ = p.parseIfStmt()
		case token.LBRACE:
			else_ = p.parseBlockStmt()
			p.expectSemi()
		default:
			p.errorExpected(p.pos, "if statement or block")
			else_ = &ast.BadStmt{From: p.pos, To: p.pos}
		}
	} else {
		p.expectSemi()
	}

	return &ast.IfStmt{If: pos, Init: init, Cond: cond, Body: body, Else: else_}
}

func (p *parser) parseTypeList() (list []ast.Expr) {
	if p.trace {
		defer un(trace(p, "TypeList"))
	}

	list = append(list, p.parseType())
	for p.tok == token.COMMA {
		p.next()
		list = append(list, p.parseType())
	}

	return
}

func (p *parser) parseCaseClause(typeSwitch bool) *ast.CaseClause {
	if p.trace {
		defer un(trace(p, "CaseClause"))
	}

	pos := p.pos
	var list []ast.Expr
	if p.tok == token.CASE {
		p.next()
		if typeSwitch {
			list = p.parseTypeList()
		} else {
			list = p.parseRhsList()
		}
	} else {
		p.expect(token.DEFAULT)
	}

	colon := p.expect(token.COLON)
	p.openScope()
	body := p.parseStmtList()
	p.closeScope()

	return &ast.CaseClause{Case: pos, List: list, Colon: colon, Body: body}
}

func isTypeSwitchAssert(x ast.Expr) bool {
	a, ok := x.(*ast.TypeAssertExpr)
	return ok && a.Type == nil
}

func (p *parser) isTypeSwitchGuard(s ast.Stmt) bool {
	switch t := s.(type) {

x.(type)

		return isTypeSwitchAssert(t.X)

v := x.(type)

		if len(t.Lhs) == 1 && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0]) {
			switch t.Tok {

permit v = x.(type) but complain

				p.error(t.TokPos, "expected ':=', found '='")
				fallthrough
			case token.DEFINE:
				return true
			}
		}
	}
	return false
}

func (p *parser) parseSwitchStmt() ast.Stmt {
	if p.trace {
		defer un(trace(p, "SwitchStmt"))
	}

	pos := p.expect(token.SWITCH)
	p.openScope()
	defer p.closeScope()

	var s1, s2 ast.Stmt
	if p.tok != token.LBRACE {
		prevLev := p.exprLev
		p.exprLev = -1
		if p.tok != token.SEMICOLON {
			s2, _ = p.parseSimpleStmt(basic)
		}
		if p.tok == token.SEMICOLON {
			p.next()
			s1 = s2
			s2 = nil

A TypeSwitchGuard may declare a variable in addition to the variable declared in the initial SimpleStmt. Introduce extra scope to avoid redeclaration errors: switch t := 0; t := x.(T) { ... } (this code is not valid Go because the first t cannot be accessed and thus is never used, the extra scope is needed for the correct error message). If we don't have a type switch, s2 must be an expression. Having the extra nested but empty scope won't affect it.

				p.openScope()
				defer p.closeScope()
				s2, _ = p.parseSimpleStmt(basic)
			}
		}
		p.exprLev = prevLev
	}

	typeSwitch := p.isTypeSwitchGuard(s2)
	lbrace := p.expect(token.LBRACE)
	var list []ast.Stmt
	for p.tok == token.CASE || p.tok == token.DEFAULT {
		list = append(list, p.parseCaseClause(typeSwitch))
	}
	rbrace := p.expect(token.RBRACE)
	p.expectSemi()
	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}

	if typeSwitch {
		return &ast.TypeSwitchStmt{Switch: pos, Init: s1, Assign: s2, Body: body}
	}

	return &ast.SwitchStmt{Switch: pos, Init: s1, Tag: p.makeExpr(s2, "switch expression"), Body: body}
}

func (p *parser) parseCommClause() *ast.CommClause {
	if p.trace {
		defer un(trace(p, "CommClause"))
	}

	p.openScope()
	pos := p.pos
	var comm ast.Stmt
	if p.tok == token.CASE {
		p.next()
		lhs := p.parseLhsList()

SendStmt

			if len(lhs) > 1 {

continue with first expression

			}
			arrow := p.pos
			p.next()
			rhs := p.parseRhs()
			comm = &ast.SendStmt{Chan: lhs[0], Arrow: arrow, Value: rhs}

RecvStmt

RecvStmt with assignment

				if len(lhs) > 2 {

continue with first two expressions

					lhs = lhs[0:2]
				}
				pos := p.pos
				p.next()
				rhs := p.parseRhs()
				as := &ast.AssignStmt{Lhs: lhs, TokPos: pos, Tok: tok, Rhs: []ast.Expr{rhs}}
				if tok == token.DEFINE {
					p.shortVarDecl(as, lhs)
				}
				comm = as

lhs must be single receive operation

				if len(lhs) > 1 {

continue with first expression

				}
				comm = &ast.ExprStmt{X: lhs[0]}
			}
		}
	} else {
		p.expect(token.DEFAULT)
	}

	colon := p.expect(token.COLON)
	body := p.parseStmtList()
	p.closeScope()

	return &ast.CommClause{Case: pos, Comm: comm, Colon: colon, Body: body}
}

func (p *parser) parseSelectStmt() *ast.SelectStmt {
	if p.trace {
		defer un(trace(p, "SelectStmt"))
	}

	pos := p.expect(token.SELECT)
	lbrace := p.expect(token.LBRACE)
	var list []ast.Stmt
	for p.tok == token.CASE || p.tok == token.DEFAULT {
		list = append(list, p.parseCommClause())
	}
	rbrace := p.expect(token.RBRACE)
	p.expectSemi()
	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}

	return &ast.SelectStmt{Select: pos, Body: body}
}

func (p *parser) parseForStmt() ast.Stmt {
	if p.trace {
		defer un(trace(p, "ForStmt"))
	}

	pos := p.expect(token.FOR)
	p.openScope()
	defer p.closeScope()

	var s1, s2, s3 ast.Stmt
	var isRange bool
	if p.tok != token.LBRACE {
		prevLev := p.exprLev
		p.exprLev = -1
		if p.tok != token.SEMICOLON {

"for range x" (nil lhs in assignment)

				pos := p.pos
				p.next()
				y := []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
				s2 = &ast.AssignStmt{Rhs: y}
				isRange = true
			} else {
				s2, isRange = p.parseSimpleStmt(rangeOk)
			}
		}
		if !isRange && p.tok == token.SEMICOLON {
			p.next()
			s1 = s2
			s2 = nil
			if p.tok != token.SEMICOLON {
				s2, _ = p.parseSimpleStmt(basic)
			}
			p.expectSemi()
			if p.tok != token.LBRACE {
				s3, _ = p.parseSimpleStmt(basic)
			}
		}
		p.exprLev = prevLev
	}

	body := p.parseBlockStmt()
	p.expectSemi()

	if isRange {

check lhs

		var key, value ast.Expr
		switch len(as.Lhs) {

nothing to do

		case 1:
			key = as.Lhs[0]
		case 2:
			key, value = as.Lhs[0], as.Lhs[1]
		default:
			p.errorExpected(as.Lhs[len(as.Lhs)-1].Pos(), "at most 2 expressions")
			return &ast.BadStmt{From: pos, To: p.safePos(body.End())}

parseSimpleStmt returned a right-hand side that is a single unary expression of the form "range x"

		x := as.Rhs[0].(*ast.UnaryExpr).X
		return &ast.RangeStmt{
			For:    pos,
			Key:    key,
			Value:  value,
			TokPos: as.TokPos,
			Tok:    as.Tok,
			X:      x,
			Body:   body,
		}
	}

regular for statement

	return &ast.ForStmt{
		For:  pos,
		Init: s1,
		Cond: p.makeExpr(s2, "boolean or range expression"),
		Post: s3,
		Body: body,
	}
}

func (p *parser) parseStmt() (s ast.Stmt) {
	if p.trace {
		defer un(trace(p, "Statement"))
	}

	switch p.tok {
	case token.CONST, token.TYPE, token.VAR:
		s = &ast.DeclStmt{Decl: p.parseDecl(stmtStart)}

tokens that may start an expression

		token.IDENT, token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operands
		token.LBRACK, token.STRUCT, token.MAP, token.CHAN, token.INTERFACE, // composite types
		token.ADD, token.SUB, token.MUL, token.AND, token.XOR, token.ARROW, token.NOT: // unary operators

because of the required look-ahead, labeled statements are parsed by parseSimpleStmt - don't expect a semicolon after them

		if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
			p.expectSemi()
		}
	case token.GO:
		s = p.parseGoStmt()
	case token.DEFER:
		s = p.parseDeferStmt()
	case token.RETURN:
		s = p.parseReturnStmt()
	case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
		s = p.parseBranchStmt(p.tok)
	case token.LBRACE:
		s = p.parseBlockStmt()
		p.expectSemi()
	case token.IF:
		s = p.parseIfStmt()
	case token.SWITCH:
		s = p.parseSwitchStmt()
	case token.SELECT:
		s = p.parseSelectStmt()
	case token.FOR:
		s = p.parseForStmt()

Is it ever possible to have an implicit semicolon producing an empty statement in a valid program? (handle correctly anyway)

		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: p.lit == "\n"}
		p.next()

a semicolon may be omitted before a closing "}"

		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: true}

no statement found

		pos := p.pos
		p.errorExpected(pos, "statement")
		p.advance(stmtStart)
		s = &ast.BadStmt{From: pos, To: p.pos}
	}

	return
}

---------------------------------------------------------------------------- Declarations


type parseSpecFunction func(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec

func isValidImport(lit string) bool {
	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
	s, _ := strconv.Unquote(lit) // go/scanner returns a legal string literal
	for _, r := range s {
		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
			return false
		}
	}
	return s != ""
}

func (p *parser) parseImportSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
	if p.trace {
		defer un(trace(p, "ImportSpec"))
	}

	var ident *ast.Ident
	switch p.tok {
	case token.PERIOD:
		ident = &ast.Ident{NamePos: p.pos, Name: "."}
		p.next()
	case token.IDENT:
		ident = p.parseIdent()
	}

	pos := p.pos
	var path string
	if p.tok == token.STRING {
		path = p.lit
		if !isValidImport(path) {
			p.error(pos, "invalid import path: "+path)
		}
		p.next()
	} else {
		p.expect(token.STRING) // use expect() error handling
	}
	p.expectSemi() // call before accessing p.linecomment

collect imports

	spec := &ast.ImportSpec{
		Doc:     doc,
		Name:    ident,
		Path:    &ast.BasicLit{ValuePos: pos, Kind: token.STRING, Value: path},
		Comment: p.lineComment,
	}
	p.imports = append(p.imports, spec)

	return spec
}

func (p *parser) parseValueSpec(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec {
	if p.trace {
		defer un(trace(p, keyword.String()+"Spec"))
	}

	pos := p.pos
	idents := p.parseIdentList()
	typ := p.tryType()

always permit optional initialization for more tolerant parsing

	if p.tok == token.ASSIGN {
		p.next()
		values = p.parseRhsList()
	}
	p.expectSemi() // call before accessing p.linecomment

	switch keyword {
	case token.VAR:
		if typ == nil && values == nil {
			p.error(pos, "missing variable type or initialization")
		}
	case token.CONST:
		if values == nil && (iota == 0 || typ != nil) {
			p.error(pos, "missing constant value")
		}
	}

Go spec: The scope of a constant or variable identifier declared inside a function begins at the end of the ConstSpec or VarSpec and ends at the end of the innermost containing block. (Global identifiers are resolved in a separate phase after parsing.)

	spec := &ast.ValueSpec{
		Doc:     doc,
		Names:   idents,
		Type:    typ,
		Values:  values,
		Comment: p.lineComment,
	}
	kind := ast.Con
	if keyword == token.VAR {
		kind = ast.Var
	}
	p.declare(spec, iota, p.topScope, kind, idents...)

	return spec
}

func (p *parser) parseTypeSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
	if p.trace {
		defer un(trace(p, "TypeSpec"))
	}

	ident := p.parseIdent()

Go spec: The scope of a type identifier declared inside a function begins at the identifier in the TypeSpec and ends at the end of the innermost containing block. (Global identifiers are resolved in a separate phase after parsing.)

	spec := &ast.TypeSpec{Doc: doc, Name: ident}
	p.declare(spec, nil, p.topScope, ast.Typ, ident)
	if p.tok == token.ASSIGN {
		spec.Assign = p.pos
		p.next()
	}
	spec.Type = p.parseType()
	p.expectSemi() // call before accessing p.linecomment
	spec.Comment = p.lineComment

	return spec
}

func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
	if p.trace {
		defer un(trace(p, "GenDecl("+keyword.String()+")"))
	}

	doc := p.leadComment
	pos := p.expect(keyword)
	var lparen, rparen token.Pos
	var list []ast.Spec
	if p.tok == token.LPAREN {
		lparen = p.pos
		p.next()
		for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
			list = append(list, f(p.leadComment, keyword, iota))
		}
		rparen = p.expect(token.RPAREN)
		p.expectSemi()
	} else {
		list = append(list, f(nil, keyword, 0))
	}

	return &ast.GenDecl{
		Doc:    doc,
		TokPos: pos,
		Tok:    keyword,
		Lparen: lparen,
		Specs:  list,
		Rparen: rparen,
	}
}

func (p *parser) parseFuncDecl() *ast.FuncDecl {
	if p.trace {
		defer un(trace(p, "FunctionDecl"))
	}

	doc := p.leadComment
	pos := p.expect(token.FUNC)
	scope := ast.NewScope(p.topScope) // function scope

	var recv *ast.FieldList
	if p.tok == token.LPAREN {
		recv = p.parseParameters(scope, false)
	}

	ident := p.parseIdent()

	params, results := p.parseSignature(scope)

	var body *ast.BlockStmt
	if p.tok == token.LBRACE {
		body = p.parseBody(scope)
		p.expectSemi()
	} else if p.tok == token.SEMICOLON {
		p.next()

opening { of function declaration on next line

			p.error(p.pos, "unexpected semicolon or newline before {")
			body = p.parseBody(scope)
			p.expectSemi()
		}
	} else {
		p.expectSemi()
	}

	decl := &ast.FuncDecl{
		Doc:  doc,
		Recv: recv,
		Name: ident,
		Type: &ast.FuncType{
			Func:    pos,
			Params:  params,
			Results: results,
		},
		Body: body,
	}

Go spec: The scope of an identifier denoting a constant, type, variable, or function (but not method) declared at top level (outside any function) is the package block. init() functions cannot be referred to and there may be more than one - don't put them in the pkgScope

		if ident.Name != "init" {
			p.declare(decl, nil, p.pkgScope, ast.Fun, ident)
		}
	}

	return decl
}

func (p *parser) parseDecl(sync map[token.Token]bool) ast.Decl {
	if p.trace {
		defer un(trace(p, "Declaration"))
	}

	var f parseSpecFunction
	switch p.tok {
	case token.CONST, token.VAR:
		f = p.parseValueSpec

	case token.TYPE:
		f = p.parseTypeSpec

	case token.FUNC:
		return p.parseFuncDecl()

	default:
		pos := p.pos
		p.errorExpected(pos, "declaration")
		p.advance(sync)
		return &ast.BadDecl{From: pos, To: p.pos}
	}

	return p.parseGenDecl(p.tok, f)
}

---------------------------------------------------------------------------- Source files


func (p *parser) parseFile() *ast.File {
	if p.trace {
		defer un(trace(p, "File"))
	}

Don't bother parsing the rest if we had errors scanning the first token. Likely not a Go source file at all.

	if p.errors.Len() != 0 {
		return nil
	}

package clause

	doc := p.leadComment

Go spec: The package clause is not a declaration; the package name does not appear in any scope.

	ident := p.parseIdent()
	if ident.Name == "_" && p.mode&DeclarationErrors != 0 {
		p.error(p.pos, "invalid package name _")
	}
	p.expectSemi()

Don't bother parsing the rest if we had errors parsing the package clause. Likely not a Go source file at all.

	if p.errors.Len() != 0 {
		return nil
	}

	p.openScope()
	p.pkgScope = p.topScope
	var decls []ast.Decl

import decls

		for p.tok == token.IMPORT {
			decls = append(decls, p.parseGenDecl(token.IMPORT, p.parseImportSpec))
		}

rest of package body

			for p.tok != token.EOF {
				decls = append(decls, p.parseDecl(declStart))
			}
		}
	}
	p.closeScope()
	assert(p.topScope == nil, "unbalanced scopes")
	assert(p.labelScope == nil, "unbalanced label scopes")

resolve global identifiers within the same file

	i := 0

i <= index for current ident

		assert(ident.Obj == unresolved, "object already resolved")
		ident.Obj = p.pkgScope.Lookup(ident.Name) // also removes unresolved sentinel
		if ident.Obj == nil {
			p.unresolved[i] = ident
			i++
		}
	}

	return &ast.File{
		Doc:        doc,
		Package:    pos,
		Name:       ident,
		Decls:      decls,
		Scope:      p.pkgScope,
		Imports:    p.imports,
		Unresolved: p.unresolved[0:i],
		Comments:   p.comments,
	}