[dev.typeparams] cmd/compile/internal/types2: disallow "free" type parameter as RHS of a type declaration

For #45639.

Change-Id: I20e331b04f464db81e916af75f70ec8ae73eb989
Reviewed-on: https://go-review.googlesource.com/c/go/+/332411
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

2 files changed, 42 insertions, 20 deletions

diff --git a/src/cmd/compile/internal/types2/testdata/examples/types.go2 b/src/cmd/compile/internal/types2/testdata/examples/types.go2
index 66e7a7b90e..4ecc34dfa4 100644
--- a/src/cmd/compile/internal/types2/testdata/examples/types.go2
+++ b/src/cmd/compile/internal/types2/testdata/examples/types.go2
@@ -155,30 +155,40 @@ type _ struct {
 	List /* ERROR List redeclared */ [int]
 }
 
+// Issue #45639: We don't allow this anymore. Keep this code
+//               in case we decide to revisit this decision.
+//
 // It's possible to declare local types whose underlying types
 // are type parameters. As with ordinary type definitions, the
 // types underlying properties are "inherited" but the methods
 // are not.
-func _[T interface{ m(); ~int }]() {
-	type L T
-	var x L
-
-	// m is not defined on L (it is not "inherited" from
-	// its underlying type).
-	x.m /* ERROR x.m undefined */ ()
-
-	// But the properties of T, such that as that it supports
-	// the operations of the types given by its type bound,
-	// are also the properties of L.
-	x++
-	_ = x - x
-
-	// On the other hand, if we define a local alias for T,
-	// that alias stands for T as expected.
-	type A = T
-	var y A
-	y.m()
-	_ = y < 0
+// func _[T interface{ m(); ~int }]() {
+// 	type L T
+// 	var x L
+// 
+// 	// m is not defined on L (it is not "inherited" from
+// 	// its underlying type).
+// 	x.m /* ERROR x.m undefined */ ()
+// 
+// 	// But the properties of T, such that as that it supports
+// 	// the operations of the types given by its type bound,
+// 	// are also the properties of L.
+// 	x++
+// 	_ = x - x
+// 
+// 	// On the other hand, if we define a local alias for T,
+// 	// that alias stands for T as expected.
+// 	type A = T
+// 	var y A
+// 	y.m()
+// 	_ = y < 0
+// }
+
+// It is not permitted to declare a local type whose underlying
+// type is a type parameters 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
 }
 
 // As a special case, an explicit type argument may be omitted
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue45639.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue45639.go2
new file mode 100644
index 0000000000..441fb4cb34
--- /dev/null
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue45639.go2
@@ -0,0 +1,12 @@
+// 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 P
+
+// It is not permitted to declare a local type whose underlying
+// type is a type parameters 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
+}