go/types: guard against checking instantiation when generics is disabled

When type checking t[_], where t is a type name, it was possible to leak
an error message related to generics. Fix this by guarding on
typeparams.Enabled.

In order to test this fix, we need to be able to run the new go/types
test only if type parameters are disabled. Introduce the .go1 test data
suffix (similar to .go2) to control this behavior.

Originally found via fuzzing, though the test case was manually
simplified.

Updates #46404

Change-Id: Ib1e2c27cf974c2a5ca5b9d6d01b84a30ba4d583b
Reviewed-on: https://go-review.googlesource.com/c/go/+/329793
Trust: Robert Findley <[email protected]>
Trust: Robert Griesemer <[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/check_test.go

@@ -207,12 +207,15 @@ func checkFiles(t *testing.T, sizes Sizes, goVersion string, filenames []string,
 		t.Fatal("no source files")
 	}
 
+	if strings.HasSuffix(filenames[0], ".go2") && !typeparams.Enabled {
+		t.Skip("type params are not enabled")
+	}
+	if strings.HasSuffix(filenames[0], ".go1") && typeparams.Enabled {
+		t.Skip("type params are enabled")
+	}
+
 	mode := parser.AllErrors
-	if strings.HasSuffix(filenames[0], ".go2") {
-		if !typeparams.Enabled {
-			t.Skip("type params are not enabled")
-		}
-	} else {
+	if !strings.HasSuffix(filenames[0], ".go2") {
 		mode |= typeparams.DisallowParsing
 	}
 

src/go/types/testdata/fixedbugs/issue46404.go1

@@ -0,0 +1,8 @@
+// Copyright 2021 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.
+
+package issue46404
+
+// Check that we don't type check t[_] as an instantiation.
+type t [t /* ERROR not a type */ [_]]_ // ERROR cannot use

src/go/types/typexpr.go

@@ -463,8 +463,12 @@ func (check *Checker) typInternal(e0 ast.Expr, def *Named) (T Type) {
 		}
 
 	case *ast.IndexExpr:
-		exprs := typeparams.UnpackExpr(e.Index)
-		return check.instantiatedType(e.X, exprs, def)
+		if typeparams.Enabled {
+			exprs := typeparams.UnpackExpr(e.Index)
+			return check.instantiatedType(e.X, exprs, def)
+		}
+		check.errorf(e0, _NotAType, "%s is not a type", e0)
+		check.use(e.X)
 
 	case *ast.ParenExpr:
 		// Generic types must be instantiated before they can be used in any form.