go/types: allow composite literals of type parameter type

This is a port of CL 342690 to go/types.

Change-Id: I27dcde237e400a84c3394a3579805014777830bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/346432
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

@@ -1184,7 +1184,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 			goto Error
 		}
 
-		switch utyp := under(base).(type) {
+		switch utyp := optype(base).(type) {
 		case *Struct:
 			if len(e.Elts) == 0 {
 				break

src/go/types/testdata/examples/types.go2

@@ -191,7 +191,7 @@ 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
@@ -294,3 +294,18 @@ func _[T interface {~int|~float64}]() {
 	_ = 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}, {}}
+}