cmd/compile/internal/types2: allow composite literals of type parameter type

Change-Id: Iaaa2a3b462da6b121f13a10595950a8502b5f271
Reviewed-on: https://go-review.googlesource.com/c/go/+/342690
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

2 files changed, 18 insertions, 2 deletions

diff --git a/src/cmd/compile/internal/types2/expr.go b/src/cmd/compile/internal/types2/expr.go
index 6d8b423714..d108093dac 100644
--- a/src/cmd/compile/internal/types2/expr.go
+++ b/src/cmd/compile/internal/types2/expr.go
@@ -1214,7 +1214,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 			goto Error
 		}
 
-		switch utyp := under(base).(type) {
+		switch utyp := optype(base).(type) {
 		case *Struct:
 			if len(e.ElemList) == 0 {
 				break
diff --git a/src/cmd/compile/internal/types2/testdata/examples/types.go2 b/src/cmd/compile/internal/types2/testdata/examples/types.go2
index d662444ead..9ee014452c 100644
--- a/src/cmd/compile/internal/types2/testdata/examples/types.go2
+++ b/src/cmd/compile/internal/types2/testdata/examples/types.go2
@@ -185,7 +185,7 @@ type _ struct {
 // }
 
 // It is not permitted to declare a local type whose underlying
-// type is a type parameters not declared by that type declaration.
+// type is a type parameter not declared by that type declaration.
 func _[T any]() {
 	type _ T         // ERROR cannot use function type parameter T as RHS in type declaration
 	type _ [_ any] T // ERROR cannot use function type parameter T as RHS in type declaration
@@ -287,3 +287,19 @@ func _[T interface{~int|~float64}]() {
 	var _ T = 1
 	_ = T(0)
 }
+
+// It is possible to create composite literals of type parameter
+// type as long as it's possible to create a composite literal
+// of the structural type of the type parameter's constraint.
+func _[P interface{ ~[]int }]() P {
+	return P{}
+	return P{1, 2, 3}
+}
+
+func _[P interface{ ~[]E }, E interface{ map[string]P } ]() P {
+	x := P{}
+	return P{{}}
+	return P{E{}}
+	return P{E{"foo": x}}
+	return P{{"foo": x}, {}}
+}