cmd/compile/internal/types2: comp literals to accept type sets with single underlying types

Change-Id: Ib51bcdf023910d244739ec176880a16e700851e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/357915
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
diff --git a/src/cmd/compile/internal/types2/expr.go b/src/cmd/compile/internal/types2/expr.go
index 1001554..3a39de7 100644
--- a/src/cmd/compile/internal/types2/expr.go
+++ b/src/cmd/compile/internal/types2/expr.go
@@ -1253,7 +1253,7 @@
 			goto Error
 		}
 
-		switch utyp := optype(base).(type) {
+		switch utyp := singleUnder(base).(type) {
 		case *Struct:
 			if len(e.ElemList) == 0 {
 				break
diff --git a/src/cmd/compile/internal/types2/testdata/check/compliterals.go2 b/src/cmd/compile/internal/types2/testdata/check/compliterals.go2
new file mode 100644
index 0000000..60eac97
--- /dev/null
+++ b/src/cmd/compile/internal/types2/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}}
+}