[dev.typeparams] go/types: embedding stand-alone type parameters is not permitted

This is a port of CL 334151 to go/types.

Fixes #47127

Change-Id: I57d69c498d2649a9e1657559e4c0271333096c88
Reviewed-on: https://go-review.googlesource.com/c/go/+/335082
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/testdata/check/issues.go2

@@ -239,7 +239,7 @@ func _[T interface{ ~func() }](f T) {
 
 type sliceOf[E any] interface{ ~[]E }
 
-func append[T interface{}, S sliceOf[T], T2 interface{ T }](s S, t ...T2) S
+func append[T interface{}, S sliceOf[T], T2 interface{}](s S, t ...T2) S
 
 var f           func()
 var cancelSlice []context.CancelFunc

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

@@ -11,19 +11,20 @@ type any interface{}
 // TODO(rFindley) the below partially applied function types should probably
 //                not be permitted (spec question).
 
-func f0[A any, B interface{~C}, C interface{~D}, D interface{~A}](A, B, C, D)
-func _() {
-	f := f0[string]
-	f("a", "b", "c", "d")
-	f0("a", "b", "c", "d")
-}
-
-func f1[A any, B interface{~A}](A, B)
-func _() {
-	f := f1[int]
-	f(int(0), int(0))
-	f1(int(0), int(0))
-}
+// Embedding stand-alone type parameters is not permitted for now. Disabled.
+// func f0[A any, B interface{~C}, C interface{~D}, D interface{~A}](A, B, C, D)
+// func _() {
+// 	f := f0[string]
+// 	f("a", "b", "c", "d")
+// 	f0("a", "b", "c", "d")
+// }
+//
+// func f1[A any, B interface{~A}](A, B)
+// func _() {
+// 	f := f1[int]
+// 	f(int(0), int(0))
+// 	f1(int(0), int(0))
+// }
 
 func f2[A any, B interface{~[]A}](A, B)
 func _() {
@@ -32,13 +33,14 @@ func _() {
 	f2(byte(0), []byte{})
 }
 
-func f3[A any, B interface{~C}, C interface{~*A}](A, B, C)
-func _() {
-	f := f3[int]
-	var x int
-	f(x, &x, &x)
-	f3(x, &x, &x)
-}
+// Embedding stand-alone type parameters is not permitted for now. Disabled.
+// func f3[A any, B interface{~C}, C interface{~*A}](A, B, C)
+// func _() {
+// 	f := f3[int]
+// 	var x int
+// 	f(x, &x, &x)
+// 	f3(x, &x, &x)
+// }
 
 func f4[A any, B interface{~[]C}, C interface{~*A}](A, B, C)
 func _() {

src/go/types/testdata/fixedbugs/issue39634.go2

@@ -31,9 +31,10 @@ type x7[A any] struct{ foo7 }
 func main7() { var _ foo7 = x7[int]{} }
 
 // crash 8
-type foo8[A any] interface { ~A }
-func bar8[A foo8[A]](a A) {}
-func main8() {}
+// Embedding stand-alone type parameters is not permitted for now. Disabled.
+// type foo8[A any] interface { ~A }
+// func bar8[A foo8[A]](a A) {}
+// func main8() {}
 
 // crash 9
 type foo9[A any] interface { foo9 /* ERROR illegal cycle */ [A] }

src/go/types/testdata/fixedbugs/issue39680.go2

@@ -4,6 +4,9 @@
 
 package p
 
+// Embedding stand-alone type parameters is not permitted for now. Disabled.
+
+/*
 import "fmt"
 
 // Minimal test case.
@@ -25,3 +28,4 @@ func Print[T constr[T]](s []T) {
 func f() {
 	Print([]string{"Hello, ", "playground\n"})
 }
+*/

src/go/types/testdata/fixedbugs/issue39948.go2

@@ -2,14 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// TODO(rfindley) Eventually, once we disallow type lists, we need to
-//                adjust this code: for 1.17 we don't accept type parameters,
-//                and for 1.18 this code is valid.
-//                Leaving for now so we can see that existing errors
-//                are being reported.
-
-package go1_17 // don't permit non-interface elements in interfaces
+package p
 
 type T[P any] interface{
-	P // ERROR P is a type parameter, not an interface
+	P // ERROR cannot embed a type parameter
 }

src/go/types/testdata/fixedbugs/issue47127.go2

@@ -0,0 +1,37 @@
+// 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.
+
+// Embedding of stand-alone type parameters is not permitted.
+
+package p
+
+type (
+        _[P any] interface{ *P | []P | chan P | map[string]P }
+        _[P any] interface{ P /* ERROR "cannot embed a type parameter" */ }
+        _[P any] interface{ ~P /* ERROR "cannot embed a type parameter" */ }
+        _[P any] interface{ int | P /* ERROR "cannot embed a type parameter" */ }
+        _[P any] interface{ int | ~P /* ERROR "cannot embed a type parameter" */ }
+)
+
+func _[P any]() {
+        type (
+                _[P any] interface{ *P | []P | chan P | map[string]P }
+                _[P any] interface{ P /* ERROR "cannot embed a type parameter" */ }
+                _[P any] interface{ ~P /* ERROR "cannot embed a type parameter" */ }
+                _[P any] interface{ int | P /* ERROR "cannot embed a type parameter" */ }
+                _[P any] interface{ int | ~P /* ERROR "cannot embed a type parameter" */ }
+
+                _ interface{ *P | []P | chan P | map[string]P }
+                _ interface{ P /* ERROR "cannot embed a type parameter" */ }
+                _ interface{ ~P /* ERROR "cannot embed a type parameter" */ }
+                _ interface{ int | P /* ERROR "cannot embed a type parameter" */ }
+                _ interface{ int | ~P /* ERROR "cannot embed a type parameter" */ }
+        )
+}
+
+func _[P any, Q interface{ *P | []P | chan P | map[string]P }]()
+func _[P any, Q interface{ P /* ERROR "cannot embed a type parameter" */ }]()
+func _[P any, Q interface{ ~P /* ERROR "cannot embed a type parameter" */ }]()
+func _[P any, Q interface{ int | P /* ERROR "cannot embed a type parameter" */ }]()
+func _[P any, Q interface{ int | ~P /* ERROR "cannot embed a type parameter" */ }]()

src/go/types/typeset.go

@@ -217,11 +217,9 @@ func computeTypeSet(check *Checker, pos token.Pos, ityp *Interface) *TypeSet {
 			//           interface before go1.18.
 			types = typ
 		case *TypeParam:
-			if check != nil && !check.allowVersion(check.pkg, 1, 18) {
-				check.errorf(atPos(pos), _InvalidIfaceEmbed, "%s is a type parameter, not an interface", typ)
-				continue
-			}
-			types = typ
+			// Embedding stand-alone type parameters is not permitted for now.
+			// This case is handled during union parsing.
+			unreachable()
 		default:
 			if typ == Typ[Invalid] {
 				continue

src/go/types/union.go

@@ -131,13 +131,18 @@ func parseUnion(check *Checker, tlist []ast.Expr) Type {
 	return newUnion(types, tilde)
 }
 
-func parseTilde(check *Checker, x ast.Expr) (Type, bool) {
-	tilde := false
+func parseTilde(check *Checker, x ast.Expr) (typ Type, tilde bool) {
 	if op, _ := x.(*ast.UnaryExpr); op != nil && op.Op == token.TILDE {
 		x = op.X
 		tilde = true
 	}
-	return check.anyType(x), tilde
+	typ = check.anyType(x)
+	// embedding stand-alone type parameters is not permitted (issue #47127).
+	if _, ok := under(typ).(*TypeParam); ok {
+		check.error(x, _Todo, "cannot embed a type parameter")
+		typ = Typ[Invalid]
+	}
+	return
 }
 
 // intersect computes the intersection of the types x and y,