[dev.typeparams] cmd/compile/internal/syntax: accept embedded type literals

The parser accepted embedded elements but the first term
of an element had to be a ~-term or a type name. This CL
fixes that.

Change-Id: I013b6cdc5963fb228867ca6597f9139db2be7ec5
Reviewed-on: https://go-review.googlesource.com/c/go/+/321109
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/syntax/parser.go

@@ -1443,6 +1443,18 @@ func (p *parser) interfaceType() *InterfaceType {
 				}
 				return false
 			}
+
+		default:
+			if p.mode&AllowGenerics != 0 {
+				pos := p.pos()
+				if t := p.typeOrNil(); t != nil {
+					f := new(Field)
+					f.pos = pos
+					f.Type = t
+					typ.MethodList = append(typ.MethodList, p.embeddedElem(f))
+					return false
+				}
+			}
 		}
 
 		if p.mode&AllowGenerics != 0 {

src/cmd/compile/internal/syntax/testdata/interface.go2

@@ -25,7 +25,6 @@ type _ interface {
 	~int | ~string
 }
 
-
 type _ interface {
 	m()
 	~int
@@ -34,3 +33,48 @@ type _ interface {
 	~int | ~string
 	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
+}