[dev.typeparams] go/parser: accept embedded type literals

This is an approximate port of CL 321109 to go/parser, though go/parser
does not have the same internal APIs as cmd/compile/internal/syntax, so
this CL required some refactoring.

Change-Id: I146ef530c969d61bab99f98f4de94b862e103ddc
Reviewed-on: https://go-review.googlesource.com/c/go/+/325703
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/parser/parser.go

@@ -1071,10 +1071,13 @@ func (p *parser) parseInterfaceType() *ast.InterfaceType {
 
 	pos := p.expect(token.INTERFACE)
 	lbrace := p.expect(token.LBRACE)
+
 	var list []*ast.Field
-	for p.tok == token.IDENT || p.parseTypeParams() && (p.tok == token.TYPE || p.tok == token.TILDE) {
-		switch p.tok {
-		case token.IDENT:
+
+parseElements:
+	for {
+		switch {
+		case p.tok == token.IDENT:
 			f := p.parseMethodSpec()
 			if f.Names == nil && p.parseTypeParams() {
 				f = p.embeddedElem(f)
@@ -1082,12 +1085,12 @@ func (p *parser) parseInterfaceType() *ast.InterfaceType {
 			p.expectSemi()
 			f.Comment = p.lineComment
 			list = append(list, f)
-		case token.TILDE:
+		case p.tok == token.TILDE && p.parseTypeParams():
 			f := p.embeddedElem(nil)
 			p.expectSemi()
 			f.Comment = p.lineComment
 			list = append(list, f)
-		case token.TYPE:
+		case p.tok == token.TYPE && p.parseTypeParams():
 			// TODO(rfindley): remove TypeList syntax and refactor the clauses above.
 
 			// all types in a type list share the same field name "type"
@@ -1099,8 +1102,22 @@ func (p *parser) parseInterfaceType() *ast.InterfaceType {
 				list = append(list, &ast.Field{Names: name, Type: typ})
 			}
 			p.expectSemi()
+		case p.parseTypeParams():
+			if t := p.tryIdentOrType(); t != nil {
+				f := new(ast.Field)
+				f.Type = t
+				f = p.embeddedElem(f)
+				p.expectSemi()
+				f.Comment = p.lineComment
+				list = append(list, f)
+			} else {
+				break parseElements
+			}
+		default:
+			break parseElements
 		}
 	}
+
 	// TODO(rfindley): the error produced here could be improved, since we could
 	// accept a identifier, 'type', or a '}' at this point.
 	rbrace := p.expect(token.RBRACE)

src/go/parser/short_test.go

@@ -200,10 +200,12 @@ var invalids = []string{
 	`package p; func (type /* ERROR "found 'type'" */ T)(T) _()`,
 	`package p; type _[A+B, /* ERROR "expected ']'" */ ] int`,
 
-	// TODO: this error should be positioned on the ':'
+	// TODO(rfindley): this error should be positioned on the ':'
 	`package p; var a = a[[]int:[ /* ERROR "expected expression" */ ]int];`,
-	// TODO: the compiler error is better here: "cannot parenthesize embedded type"
-	`package p; type I1 interface{}; type I2 interface{ (/* ERROR "expected '}', found '\('" */ I1) }`,
+
+	// TODO(rfindley): the compiler error is better here: "cannot parenthesize embedded type"
+	// TODO(rfindley): confirm that parenthesized types should now be accepted.
+	// `package p; type I1 interface{}; type I2 interface{ (/* ERROR "expected '}', found '\('" */ I1) }`,
 
 	// issue 8656
 	`package p; func f() (a b string /* ERROR "missing ','" */ , ok bool)`,

src/go/parser/testdata/interface.go2

@@ -25,7 +25,6 @@ type _ interface {
 	~int | ~string
 }
 
-
 type _ interface {
 	m()
 	~int
@@ -35,3 +34,47 @@ type _ interface {
 	type bool, int, float64
 }
 
+type _ interface {
+	int
+	[]byte
+	[10]int
+	struct{}
+	*int
+	func()
+	interface{}
+	map[string]int
+	chan T
+	chan<- T
+	<-chan T
+	T[int]
+}
+
+type _ interface {
+	int | string
+	[]byte | string
+	[10]int | string
+	struct{} | string
+	*int | string
+	func() | string
+	interface{} | string
+	map[string]int | string
+	chan T | string
+	chan<- T | string
+	<-chan T | string
+	T[int] | string
+}
+
+type _ interface {
+	~int | string
+	~[]byte | string
+	~[10]int | string
+	~struct{} | string
+	~*int | string
+	~func() | string
+	~interface{} | string
+	~map[string]int | string
+	~chan T | string
+	~chan<- T | string
+	~<-chan T | string
+	~T[int] | string
+}