go/types: comp literals to accept type sets with single underlying types

This is a clean port of CL 357915 to go/types.

Change-Id: Idf5850a8bdcf3596c561c97bcd60539945139743
Reviewed-on: https://go-review.googlesource.com/c/go/+/359877
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/types/expr.go

@@ -1223,7 +1223,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 			goto Error
 		}
 
-		switch utyp := optype(base).(type) {
+		switch utyp := singleUnder(base).(type) {
 		case *Struct:
 			if len(e.Elts) == 0 {
 				break

src/go/types/testdata/check/compliterals.go2

@@ -0,0 +1,22 @@
+// Copyright 2012 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.
+
+// Composite literals with parameterized types
+
+package comp_literals
+
+type myStruct struct {
+	f int
+}
+
+type slice[E any] []E
+
+func struct_literals[S struct{f int}|myStruct]() {
+	_ = S{}
+	_ = S{0}
+	_ = S{f: 0}
+
+        _ = slice[int]{1, 2, 3}
+        _ = slice[S]{{}, {0}, {f:0}}
+}