github.com/q45/go@v0.0.0-20151101211701-a4fb8c13db3f/src/go/printer/testdata/parser.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// 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.

package parser

import (
	"fmt"
	"go/ast"
	"go/scanner"
	"go/token"
)

// The mode parameter to the Parse* functions is a set of flags (or 0).
// They control the amount of source code parsed and other optional
// parser functionality.
//
const (
	PackageClauseOnly uint = 1 << iota // parsing stops after package clause
	ImportsOnly                        // parsing stops after import declarations
	ParseComments                      // parse comments and add them to AST
	Trace                              // print a trace of parsed productions
	DeclarationErrors                  // report declaration errors
)

// The parser structure holds the parser's internal state.
type parser struct {
	file *token.File
	scanner.ErrorVector
	scanner scanner.Scanner

	// Tracing/debugging
	mode   uint // parsing mode
	trace  bool // == (mode & Trace != 0)
	indent uint // 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

	// Non-syntactic parser control
	exprLev int // < 0: in control clause, >= 0: in 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 scope
	// (maintained by open/close LabelScope)
	labelScope  *ast.Scope     // label scope for current function
	targetStack [][]*ast.Ident // stack of unresolved labels
}

// scannerMode returns the scanner mode bits given the parser's mode bits.
func scannerMode(mode uint) uint {
	var m uint = scanner.InsertSemis
	if mode&ParseComments != 0 {
		m |= scanner.ScanComments
	}
	return m
}

func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode uint) {
	p.file = fset.AddFile(filename, fset.Base(), len(src))
	p.scanner.Init(p.file, src, p, scannerMode(mode))

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

	p.next()

	// set up the pkgScope here (as opposed to in parseFile) because
	// there are other parser entry points (ParseExpr, etc.)
	p.openScope()
	p.pkgScope = p.topScope

	// for the same reason, set up a label scope
	p.openLabelScope()
}

// ----------------------------------------------------------------------------
// 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)
}

func (p *parser) closeLabelScope() {
	// 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 interface{}, scope *ast.Scope, kind ast.ObjKind, idents ...*ast.Ident) {
	for _, ident := range idents {
		assert(ident.Obj == nil, "identifier already declared or resolved")
		if ident.Name != "_" {
			obj := ast.NewObj(kind, ident.Name)
			// remember the corresponding declaration for redeclaration
			// errors and global variable resolution/typechecking phase
			obj.Decl = decl
			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))
			}
			ident.Obj = obj
		}
	}
}

func (p *parser) shortVarDecl(idents []*ast.Ident) {
	// 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 _, ident := range idents {
		assert(ident.Obj == nil, "identifier already declared or resolved")
		if ident.Name != "_" {
			obj := ast.NewObj(ast.Var, ident.Name)
			// short var declarations cannot have redeclaration errors
			// and are not global => no need to remember the respective
			// declaration
			alt := p.topScope.Insert(obj)
			if alt == nil {
				n++ // new declaration
				alt = obj
			}
			ident.Obj = alt
		}
	}
	if n == 0 && p.mode&DeclarationErrors != 0 {
		p.error(idents[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)

func (p *parser) resolve(x ast.Expr) {
	// 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
	ident.Obj = unresolved
	p.unresolved = append(p.unresolved, ident)
}

// ----------------------------------------------------------------------------
// Parsing support

func (p *parser) printTrace(a ...interface{}) {
	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " +
		". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
	const n = uint(len(dots))
	pos := p.file.Position(p.pos)
	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
	i := 2 * p.indent
	for ; i > n; i -= n {
		fmt.Print(dots)
	}
	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.
func (p *parser) next0() {
	// 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.
func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
	// /*-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)
	if p.lit[1] == '*' {
		// 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{p.pos, 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. An empty line or non-comment
// token terminates a comment group.
//
func (p *parser) consumeCommentGroup() (comments *ast.CommentGroup, endline int) {
	var list []*ast.Comment
	endline = p.file.Line(p.pos)
	for p.tok == token.COMMENT && endline+1 >= p.file.Line(p.pos) {
		var comment *ast.Comment
		comment, endline = p.consumeComment()
		list = append(list, comment)
	}

	// add comment group to the comments list
	comments = &ast.CommentGroup{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
// 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
	line := p.file.Line(p.pos) // current line
	p.next0()

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

		if p.file.Line(p.pos) == line {
			// 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()
			if p.file.Line(p.pos) != endline {
				// 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()
		}

		if endline+1 == p.file.Line(p.pos) {
			// 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
		}
	}
}

func (p *parser) error(pos token.Pos, msg string) {
	p.Error(p.file.Position(pos), msg)
}

func (p *parser) errorExpected(pos token.Pos, msg string) {
	msg = "expected " + msg
	if pos == p.pos {
		// the error happened at the current position;
		// make the error message more specific
		if p.tok == token.SEMICOLON && p.lit[0] == '\n' {
			msg += ", found newline"
		} else {
			msg += ", found '" + p.tok.String() + "'"
			if p.tok.IsLiteral() {
				msg += " " + p.lit
			}
		}
	}
	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
}

func (p *parser) expectSemi() {
	if p.tok != token.RPAREN && p.tok != token.RBRACE {
		p.expect(token.SEMICOLON)
	}
}

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

// ----------------------------------------------------------------------------
// 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{pos, name, nil}
}

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

	return
}

func (p *parser) parseLhsList() []ast.Expr {
	list := p.parseExprList(true)
	switch p.tok {
	case token.DEFINE:
		// lhs of a short variable declaration
		p.shortVarDecl(p.makeIdentList(list))
	case token.COLON:
		// 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
	default:
		// identifiers must be declared elsewhere
		for _, x := range list {
			p.resolve(x)
		}
	}
	return list
}

func (p *parser) parseRhsList() []ast.Expr {
	return p.parseExprList(false)
}

// ----------------------------------------------------------------------------
// 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.next() // make progress
		return &ast.BadExpr{pos, 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"))
	}

	ident := p.parseIdent()
	// don't resolve ident yet - it may be a parameter or field name

	if p.tok == token.PERIOD {
		// ident is a package name
		p.next()
		p.resolve(ident)
		sel := p.parseIdent()
		return &ast.SelectorExpr{ident, sel}
	}

	return ident
}

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

	lbrack := p.expect(token.LBRACK)
	var len ast.Expr
	if ellipsisOk && p.tok == token.ELLIPSIS {
		len = &ast.Ellipsis{p.pos, nil}
		p.next()
	} else if p.tok != token.RBRACK {
		len = p.parseRhs()
	}
	p.expect(token.RBRACK)
	elt := p.parseType()

	return &ast.ArrayType{lbrack, len, 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 {
			pos := x.(ast.Expr).Pos()
			p.errorExpected(pos, "identifier")
			ident = &ast.Ident{pos, "_", nil}
		}
		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

	// fields
	list, typ := p.parseVarList(false)

	// optional tag
	var tag *ast.BasicLit
	if p.tok == token.STRING {
		tag = &ast.BasicLit{p.pos, p.tok, p.lit}
		p.next()
	}

	// analyze case
	var idents []*ast.Ident
	if typ != nil {
		// IdentifierList Type
		idents = p.makeIdentList(list)
	} else {
		// ["*"] TypeName (AnonymousField)
		typ = list[0] // we always have at least one element
		p.resolve(typ)
		if n := len(list); n > 1 || !isTypeName(deref(typ)) {
			pos := typ.Pos()
			p.errorExpected(pos, "anonymous field")
			typ = &ast.BadExpr{pos, list[n-1].End()}
		}
	}

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

	field := &ast.Field{doc, idents, typ, tag, p.lineComment}
	p.declare(field, scope, ast.Var, idents...)

	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
	for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
		// 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)

	// TODO(gri): store struct scope in AST
	return &ast.StructType{pos, &ast.FieldList{lbrace, list, rbrace}, false}
}

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, base}
}

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

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{pos, p.pos}
	}
	return typ
}

func (p *parser) parseVarList(isParam bool) (list []ast.Expr, typ ast.Expr) {
	if p.trace {
		defer un(trace(p, "VarList"))
	}

	// a list of identifiers looks like a list of type names
	for {
		// parseVarType accepts any type (including parenthesized ones)
		// even though the syntax does not permit them here: we
		// accept them all for more robust parsing and complain
		// afterwards
		list = append(list, p.parseVarType(isParam))
		if p.tok != token.COMMA {
			break
		}
		p.next()
	}

	// if we had a list of identifiers, it must be followed by a type
	typ = p.tryVarType(isParam)
	if typ != nil {
		p.resolve(typ)
	}

	return
}

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

	list, typ := p.parseVarList(ellipsisOk)
	if typ != nil {
		// IdentifierList Type
		idents := p.makeIdentList(list)
		field := &ast.Field{nil, idents, typ, nil, nil}
		params = append(params, field)
		// Go spec: The scope of an identifier denoting a function
		// parameter or result variable is the function body.
		p.declare(field, scope, ast.Var, idents...)
		if p.tok == token.COMMA {
			p.next()
		}

		for p.tok != token.RPAREN && p.tok != token.EOF {
			idents := p.parseIdentList()
			typ := p.parseVarType(ellipsisOk)
			field := &ast.Field{nil, idents, typ, nil, nil}
			params = append(params, field)
			// Go spec: The scope of an identifier denoting a function
			// parameter or result variable is the function body.
			p.declare(field, scope, ast.Var, idents...)
			if p.tok != token.COMMA {
				break
			}
			p.next()
		}

	} else {
		// Type { "," Type } (anonymous parameters)
		params = make([]*ast.Field, len(list))
		for i, x := range list {
			p.resolve(x)
			params[i] = &ast.Field{Type: x}
		}
	}

	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{lparen, params, 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{pos, params, 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()
	if ident, isIdent := x.(*ast.Ident); isIdent && p.tok == token.LPAREN {
		// method
		idents = []*ast.Ident{ident}
		scope := ast.NewScope(nil) // method scope
		params, results := p.parseSignature(scope)
		typ = &ast.FuncType{token.NoPos, params, results}
	} else {
		// embedded interface
		typ = x
	}
	p.expectSemi() // call before accessing p.linecomment

	spec := &ast.Field{doc, idents, typ, nil, p.lineComment}
	p.declare(spec, 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)

	// TODO(gri): store interface scope in AST
	return &ast.InterfaceType{pos, &ast.FieldList{lbrace, list, rbrace}, false}
}

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{pos, key, value}
}

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

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

	return &ast.ChanType{pos, dir, value}
}

// If the result is an identifier, it is not resolved.
func (p *parser) tryIdentOrType(ellipsisOk bool) ast.Expr {
	switch p.tok {
	case token.IDENT:
		return p.parseTypeName()
	case token.LBRACK:
		return p.parseArrayType(ellipsisOk)
	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, typ, rparen}
	}

	// no type found
	return nil
}

func (p *parser) tryType() ast.Expr {
	typ := p.tryIdentOrType(false)
	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.expect(token.RBRACE)

	return &ast.BlockStmt{lbrace, list, 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.expect(token.RBRACE)

	return &ast.BlockStmt{lbrace, list, rbrace}
}

// ----------------------------------------------------------------------------
// Expressions

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

	typ, scope := p.parseFuncType()
	if p.tok != token.LBRACE {
		// function type only
		return typ
	}

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

	return &ast.FuncLit{typ, 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{p.pos, p.tok, p.lit}
		p.next()
		return x

	case token.LPAREN:
		lparen := p.pos
		p.next()
		p.exprLev++
		x := p.parseRhs()
		p.exprLev--
		rparen := p.expect(token.RPAREN)
		return &ast.ParenExpr{lparen, x, rparen}

	case token.FUNC:
		return p.parseFuncTypeOrLit()

	default:
		if typ := p.tryIdentOrType(true); typ != nil {
			// could be type for composite literal or conversion
			_, isIdent := typ.(*ast.Ident)
			assert(!isIdent, "type cannot be identifier")
			return typ
		}
	}

	pos := p.pos
	p.errorExpected(pos, "operand")
	p.next() // make progress
	return &ast.BadExpr{pos, 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, sel}
}

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

	p.expect(token.LPAREN)
	var typ ast.Expr
	if p.tok == token.TYPE {
		// type switch: typ == nil
		p.next()
	} else {
		typ = p.parseType()
	}
	p.expect(token.RPAREN)

	return &ast.TypeAssertExpr{x, typ}
}

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

	lbrack := p.expect(token.LBRACK)
	p.exprLev++
	var low, high ast.Expr
	isSlice := false
	if p.tok != token.COLON {
		low = p.parseRhs()
	}
	if p.tok == token.COLON {
		isSlice = true
		p.next()
		if p.tok != token.RBRACK {
			high = p.parseRhs()
		}
	}
	p.exprLev--
	rbrack := p.expect(token.RBRACK)

	if isSlice {
		return &ast.SliceExpr{x, lbrack, low, high, rbrack}
	}
	return &ast.IndexExpr{x, lbrack, low, 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.parseRhs())
		if p.tok == token.ELLIPSIS {
			ellipsis = p.pos
			p.next()
		}
		if p.tok != token.COMMA {
			break
		}
		p.next()
	}
	p.exprLev--
	rparen := p.expect(token.RPAREN)

	return &ast.CallExpr{fun, lparen, list, ellipsis, rparen}
}

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

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

	x := p.parseExpr(keyOk) // don't resolve if map key
	if keyOk {
		if p.tok == token.COLON {
			colon := p.pos
			p.next()
			return &ast.KeyValueExpr{x, colon, p.parseElement(false)}
		}
		p.resolve(x) // not a map key
	}

	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(true))
		if p.tok != token.COMMA {
			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.expect(token.RBRACE)
	return &ast.CompositeLit{typ, lbrace, elts, rbrace}
}

// checkExpr checks that x is an expression (and not a type).
func (p *parser) checkExpr(x ast.Expr) ast.Expr {
	switch t := 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:
	case *ast.TypeAssertExpr:
		if t.Type == nil {
			// the form X.(type) is only allowed in type switch expressions
			p.errorExpected(x.Pos(), "expression")
			x = &ast.BadExpr{x.Pos(), x.End()}
		}
	case *ast.CallExpr:
	case *ast.StarExpr:
	case *ast.UnaryExpr:
		if t.Op == token.RANGE {
			// the range operator is only allowed at the top of a for statement
			p.errorExpected(x.Pos(), "expression")
			x = &ast.BadExpr{x.Pos(), x.End()}
		}
	case *ast.BinaryExpr:
	default:
		// all other nodes are not proper expressions
		p.errorExpected(x.Pos(), "expression")
		x = &ast.BadExpr{x.Pos(), 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.UnaryExpr:
		if t.Op == token.RANGE {
			// the range operator is only allowed at the top of a for statement
			p.errorExpected(x.Pos(), "expression")
			x = &ast.BadExpr{x.Pos(), x.End()}
		}
	case *ast.ArrayType:
		if len, isEllipsis := t.Len.(*ast.Ellipsis); isEllipsis {
			p.error(len.Pos(), "expected array length, found '...'")
			x = &ast.BadExpr{x.Pos(), 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.checkExpr(x))
			case token.LPAREN:
				x = p.parseTypeAssertion(p.checkExpr(x))
			default:
				pos := p.pos
				p.next() // make progress
				p.errorExpected(pos, "selector or type assertion")
				x = &ast.BadExpr{pos, p.pos}
			}
		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, token.RANGE:
		pos, op := p.pos, p.tok
		p.next()
		x := p.parseUnaryExpr(false)
		return &ast.UnaryExpr{pos, op, p.checkExpr(x)}

	case token.ARROW:
		// channel type or receive expression
		pos := p.pos
		p.next()
		if p.tok == token.CHAN {
			p.next()
			value := p.parseType()
			return &ast.ChanType{pos, ast.RECV, value}
		}

		x := p.parseUnaryExpr(false)
		return &ast.UnaryExpr{pos, token.ARROW, p.checkExpr(x)}

	case token.MUL:
		// pointer type or unary "*" expression
		pos := p.pos
		p.next()
		x := p.parseUnaryExpr(false)
		return &ast.StarExpr{pos, p.checkExprOrType(x)}
	}

	return p.parsePrimaryExpr(lhs)
}

// 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 prec := p.tok.Precedence(); prec >= prec1; prec-- {
		for p.tok.Precedence() == prec {
			pos, op := p.pos, p.tok
			p.next()
			if lhs {
				p.resolve(x)
				lhs = false
			}
			y := p.parseBinaryExpr(false, prec+1)
			x = &ast.BinaryExpr{p.checkExpr(x), pos, op, p.checkExpr(y)}
		}
	}

	return x
}

// If lhs is set and the result is an identifier, it is not resolved.
// TODO(gri): parseExpr may return a type or even a raw type ([..]int) -
//            should reject when a type/raw type is obviously not allowed
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 {
	return p.parseExpr(false)
}

// ----------------------------------------------------------------------------
// Statements

func (p *parser) parseSimpleStmt(labelOk bool) ast.Stmt {
	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,
		token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
		// assignment statement
		pos, tok := p.pos, p.tok
		p.next()
		y := p.parseRhsList()
		return &ast.AssignStmt{x, pos, tok, y}
	}

	if len(x) > 1 {
		p.errorExpected(x[0].Pos(), "1 expression")
		// continue with first expression
	}

	switch p.tok {
	case token.COLON:
		// labeled statement
		colon := p.pos
		p.next()
		if label, isIdent := x[0].(*ast.Ident); labelOk && isIdent {
			// 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, colon, p.parseStmt()}
			p.declare(stmt, p.labelScope, ast.Lbl, label)
			return stmt
		}
		p.error(x[0].Pos(), "illegal label declaration")
		return &ast.BadStmt{x[0].Pos(), colon + 1}

	case token.ARROW:
		// send statement
		arrow := p.pos
		p.next() // consume "<-"
		y := p.parseRhs()
		return &ast.SendStmt{x[0], arrow, y}

	case token.INC, token.DEC:
		// increment or decrement
		s := &ast.IncDecStmt{x[0], p.pos, p.tok}
		p.next() // consume "++" or "--"
		return s
	}

	// expression
	return &ast.ExprStmt{x[0]}
}

func (p *parser) parseCallExpr() *ast.CallExpr {
	x := p.parseRhs()
	if call, isCall := x.(*ast.CallExpr); isCall {
		return call
	}
	p.errorExpected(x.Pos(), "function/method call")
	return nil
}

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

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

	return &ast.GoStmt{pos, call}
}

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

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

	return &ast.DeferStmt{pos, 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{pos, 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 {
		label = p.parseIdent()
		// add to list of unresolved targets
		n := len(p.targetStack) - 1
		p.targetStack[n] = append(p.targetStack[n], label)
	}
	p.expectSemi()

	return &ast.BranchStmt{pos, tok, label}
}

func (p *parser) makeExpr(s ast.Stmt) ast.Expr {
	if s == nil {
		return nil
	}
	if es, isExpr := s.(*ast.ExprStmt); isExpr {
		return p.checkExpr(es.X)
	}
	p.error(s.Pos(), "expected condition, found simple statement")
	return &ast.BadExpr{s.Pos(), s.End()}
}

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

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

	var s ast.Stmt
	var x ast.Expr
	{
		prevLev := p.exprLev
		p.exprLev = -1
		if p.tok == token.SEMICOLON {
			p.next()
			x = p.parseRhs()
		} else {
			s = p.parseSimpleStmt(false)
			if p.tok == token.SEMICOLON {
				p.next()
				x = p.parseRhs()
			} else {
				x = p.makeExpr(s)
				s = nil
			}
		}
		p.exprLev = prevLev
	}

	body := p.parseBlockStmt()
	var else_ ast.Stmt
	if p.tok == token.ELSE {
		p.next()
		else_ = p.parseStmt()
	} else {
		p.expectSemi()
	}

	return &ast.IfStmt{pos, s, x, body, 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(exprSwitch 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 exprSwitch {
			list = p.parseRhsList()
		} else {
			list = p.parseTypeList()
		}
	} else {
		p.expect(token.DEFAULT)
	}

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

	return &ast.CaseClause{pos, list, colon, body}
}

func isExprSwitch(s ast.Stmt) bool {
	if s == nil {
		return true
	}
	if e, ok := s.(*ast.ExprStmt); ok {
		if a, ok := e.X.(*ast.TypeAssertExpr); ok {
			return a.Type != nil // regular type assertion
		}
		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(false)
		}
		if p.tok == token.SEMICOLON {
			p.next()
			s1 = s2
			s2 = nil
			if p.tok != token.LBRACE {
				s2 = p.parseSimpleStmt(false)
			}
		}
		p.exprLev = prevLev
	}

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

	if exprSwitch {
		return &ast.SwitchStmt{pos, s1, p.makeExpr(s2), body}
	}
	// type switch
	// TODO(gri): do all the checks!
	return &ast.TypeSwitchStmt{pos, s1, s2, 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()
		if p.tok == token.ARROW {
			// SendStmt
			if len(lhs) > 1 {
				p.errorExpected(lhs[0].Pos(), "1 expression")
				// continue with first expression
			}
			arrow := p.pos
			p.next()
			rhs := p.parseRhs()
			comm = &ast.SendStmt{lhs[0], arrow, rhs}
		} else {
			// RecvStmt
			pos := p.pos
			tok := p.tok
			var rhs ast.Expr
			if tok == token.ASSIGN || tok == token.DEFINE {
				// RecvStmt with assignment
				if len(lhs) > 2 {
					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
					// continue with first two expressions
					lhs = lhs[0:2]
				}
				p.next()
				rhs = p.parseRhs()
			} else {
				// rhs must be single receive operation
				if len(lhs) > 1 {
					p.errorExpected(lhs[0].Pos(), "1 expression")
					// continue with first expression
				}
				rhs = lhs[0]
				lhs = nil // there is no lhs
			}
			if x, isUnary := rhs.(*ast.UnaryExpr); !isUnary || x.Op != token.ARROW {
				p.errorExpected(rhs.Pos(), "send or receive operation")
				rhs = &ast.BadExpr{rhs.Pos(), rhs.End()}
			}
			if lhs != nil {
				comm = &ast.AssignStmt{lhs, pos, tok, []ast.Expr{rhs}}
			} else {
				comm = &ast.ExprStmt{rhs}
			}
		}
	} else {
		p.expect(token.DEFAULT)
	}

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

	return &ast.CommClause{pos, comm, colon, 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, list, rbrace}

	return &ast.SelectStmt{pos, 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
	if p.tok != token.LBRACE {
		prevLev := p.exprLev
		p.exprLev = -1
		if p.tok != token.SEMICOLON {
			s2 = p.parseSimpleStmt(false)
		}
		if p.tok == token.SEMICOLON {
			p.next()
			s1 = s2
			s2 = nil
			if p.tok != token.SEMICOLON {
				s2 = p.parseSimpleStmt(false)
			}
			p.expectSemi()
			if p.tok != token.LBRACE {
				s3 = p.parseSimpleStmt(false)
			}
		}
		p.exprLev = prevLev
	}

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

	if as, isAssign := s2.(*ast.AssignStmt); isAssign {
		// possibly a for statement with a range clause; check assignment operator
		if as.Tok != token.ASSIGN && as.Tok != token.DEFINE {
			p.errorExpected(as.TokPos, "'=' or ':='")
			return &ast.BadStmt{pos, body.End()}
		}
		// check lhs
		var key, value ast.Expr
		switch len(as.Lhs) {
		case 2:
			key, value = as.Lhs[0], as.Lhs[1]
		case 1:
			key = as.Lhs[0]
		default:
			p.errorExpected(as.Lhs[0].Pos(), "1 or 2 expressions")
			return &ast.BadStmt{pos, body.End()}
		}
		// check rhs
		if len(as.Rhs) != 1 {
			p.errorExpected(as.Rhs[0].Pos(), "1 expression")
			return &ast.BadStmt{pos, body.End()}
		}
		if rhs, isUnary := as.Rhs[0].(*ast.UnaryExpr); isUnary && rhs.Op == token.RANGE {
			// rhs is range expression
			// (any short variable declaration was handled by parseSimpleStat above)
			return &ast.RangeStmt{pos, key, value, as.TokPos, as.Tok, rhs.X, body}
		}
		p.errorExpected(s2.Pos(), "range clause")
		return &ast.BadStmt{pos, body.End()}
	}

	// regular for statement
	return &ast.ForStmt{pos, s1, p.makeExpr(s2), s3, 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{p.parseDecl()}
	case
		// tokens that may start a top-level expression
		token.IDENT, token.INT, token.FLOAT, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operand
		token.LBRACK, token.STRUCT, // composite type
		token.MUL, token.AND, token.ARROW, token.ADD, token.SUB, token.XOR: // unary operators
		s = p.parseSimpleStmt(true)
		// 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()
	case token.SEMICOLON:
		s = &ast.EmptyStmt{p.pos}
		p.next()
	case token.RBRACE:
		// a semicolon may be omitted before a closing "}"
		s = &ast.EmptyStmt{p.pos}
	default:
		// no statement found
		pos := p.pos
		p.errorExpected(pos, "statement")
		p.next() // make progress
		s = &ast.BadStmt{pos, p.pos}
	}

	return
}

// ----------------------------------------------------------------------------
// Declarations

type parseSpecFunction func(p *parser, doc *ast.CommentGroup, iota int) ast.Spec

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

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

	var path *ast.BasicLit
	if p.tok == token.STRING {
		path = &ast.BasicLit{p.pos, p.tok, p.lit}
		p.next()
	} else {
		p.expect(token.STRING) // use expect() error handling
	}
	p.expectSemi() // call before accessing p.linecomment

	// collect imports
	spec := &ast.ImportSpec{doc, ident, path, p.lineComment}
	p.imports = append(p.imports, spec)

	return spec
}

func parseConstSpec(p *parser, doc *ast.CommentGroup, iota int) ast.Spec {
	if p.trace {
		defer un(trace(p, "ConstSpec"))
	}

	idents := p.parseIdentList()
	typ := p.tryType()
	var values []ast.Expr
	if typ != nil || p.tok == token.ASSIGN || iota == 0 {
		p.expect(token.ASSIGN)
		values = p.parseRhsList()
	}
	p.expectSemi() // call before accessing p.linecomment

	// 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, idents, typ, values, p.lineComment}
	p.declare(spec, p.topScope, ast.Con, idents...)

	return spec
}

func parseTypeSpec(p *parser, doc *ast.CommentGroup, _ 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, ident, nil, nil}
	p.declare(spec, p.topScope, ast.Typ, ident)

	spec.Type = p.parseType()
	p.expectSemi() // call before accessing p.linecomment
	spec.Comment = p.lineComment

	return spec
}

func parseVarSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
	if p.trace {
		defer un(trace(p, "VarSpec"))
	}

	idents := p.parseIdentList()
	typ := p.tryType()
	var values []ast.Expr
	if typ == nil || p.tok == token.ASSIGN {
		p.expect(token.ASSIGN)
		values = p.parseRhsList()
	}
	p.expectSemi() // call before accessing p.linecomment

	// 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, idents, typ, values, p.lineComment}
	p.declare(spec, p.topScope, ast.Var, idents...)

	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, p.leadComment, iota))
		}
		rparen = p.expect(token.RPAREN)
		p.expectSemi()
	} else {
		list = append(list, f(p, nil, 0))
	}

	return &ast.GenDecl{doc, pos, keyword, lparen, list, rparen}
}

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

	pos := p.pos
	par := p.parseParameters(scope, false)

	// must have exactly one receiver
	if par.NumFields() != 1 {
		p.errorExpected(pos, "exactly one receiver")
		// TODO determine a better range for BadExpr below
		par.List = []*ast.Field{{Type: &ast.BadExpr{pos, pos}}}
		return par
	}

	// recv type must be of the form ["*"] identifier
	recv := par.List[0]
	base := deref(recv.Type)
	if _, isIdent := base.(*ast.Ident); !isIdent {
		p.errorExpected(base.Pos(), "(unqualified) identifier")
		par.List = []*ast.Field{{Type: &ast.BadExpr{recv.Pos(), recv.End()}}}
	}

	return par
}

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.parseReceiver(scope)
	}

	ident := p.parseIdent()

	params, results := p.parseSignature(scope)

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

	decl := &ast.FuncDecl{doc, recv, ident, &ast.FuncType{pos, params, results}, body}
	if recv == nil {
		// 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, p.pkgScope, ast.Fun, ident)
		}
	}

	return decl
}

func (p *parser) parseDecl() ast.Decl {
	if p.trace {
		defer un(trace(p, "Declaration"))
	}

	var f parseSpecFunction
	switch p.tok {
	case token.CONST:
		f = parseConstSpec

	case token.TYPE:
		f = parseTypeSpec

	case token.VAR:
		f = parseVarSpec

	case token.FUNC:
		return p.parseFuncDecl()

	default:
		pos := p.pos
		p.errorExpected(pos, "declaration")
		p.next() // make progress
		decl := &ast.BadDecl{pos, p.pos}
		return decl
	}

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

func (p *parser) parseDeclList() (list []ast.Decl) {
	if p.trace {
		defer un(trace(p, "DeclList"))
	}

	for p.tok != token.EOF {
		list = append(list, p.parseDecl())
	}

	return
}

// ----------------------------------------------------------------------------
// Source files

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

	// package clause
	doc := p.leadComment
	pos := p.expect(token.PACKAGE)
	// 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.error(p.pos, "invalid package name _")
	}
	p.expectSemi()

	var decls []ast.Decl

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

	if p.ErrorCount() == 0 && p.mode&PackageClauseOnly == 0 {
		// import decls
		for p.tok == token.IMPORT {
			decls = append(decls, p.parseGenDecl(token.IMPORT, parseImportSpec))
		}

		if p.mode&ImportsOnly == 0 {
			// rest of package body
			for p.tok != token.EOF {
				decls = append(decls, p.parseDecl())
			}
		}
	}

	assert(p.topScope == p.pkgScope, "imbalanced scopes")

	// resolve global identifiers within the same file
	i := 0
	for _, ident := range p.unresolved {
		// 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++
		}
	}

	// TODO(gri): store p.imports in AST
	return &ast.File{doc, pos, ident, decls, p.pkgScope, p.imports, p.unresolved[0:i], p.comments}
}