go/types: better error for type assertion/switch on type parameter value

This is a port of CL 363439 from types2 to go/types.

Change-Id: Ic71871874345e1d0a4a42703e3673aadd11f2bfc
Reviewed-on: https://go-review.googlesource.com/c/go/+/364378
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: findleyr

src/go/types/expr.go

@@ -1432,6 +1432,11 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		if x.mode == invalid {
 			goto Error
 		}
+		// TODO(gri) we may want to permit type assertions on type parameter values at some point
+		if isTypeParam(x.typ) {
+			check.invalidOp(x, _InvalidAssert, "cannot use type assertion on type parameter value %s", x)
+			goto Error
+		}
 		xtyp, _ := under(x.typ).(*Interface)
 		if xtyp == nil {
 			check.invalidOp(x, _InvalidAssert, "%s is not an interface", x)

src/go/types/stmt.go

@@ -685,6 +685,11 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		if x.mode == invalid {
 			return
 		}
+		// TODO(gri) we may want to permit type switches on type parameter values at some point
+		if isTypeParam(x.typ) {
+			check.errorf(&x, _InvalidTypeSwitch, "cannot use type switch on type parameter value %s", &x)
+			return
+		}
 		xtyp, _ := under(x.typ).(*Interface)
 		if xtyp == nil {
 			check.errorf(&x, _InvalidTypeSwitch, "%s is not an interface", &x)

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

@@ -481,8 +481,8 @@ func (_ R2[X, Y]) m2(X) Y
 // type assertions and type switches over generic types lead to errors for now
 
 func _[T any](x T) {
-	_ = x /* ERROR not an interface */ .(int)
-	switch x /* ERROR not an interface */ .(type) {
+	_ = x /* ERROR cannot use type assertion */ .(int)
+	switch x /* ERROR cannot use type switch */ .(type) {
 	}
 
 	// work-around
@@ -493,8 +493,8 @@ func _[T any](x T) {
 }
 
 func _[T interface{~int}](x T) {
-	_ = x /* ERROR not an interface */ .(int)
-	switch x /* ERROR not an interface */ .(type) {
+	_ = x /* ERROR cannot use type assertion */ .(int)
+	switch x /* ERROR cannot use type switch */ .(type) {
 	}
 
 	// work-around