[dev.typeparams] cmd/compile/internal/types2: embedding stand-alone type parameters is not permitted

For #47127.

Change-Id: Ie979ff56ae7c2dd0e5ce0ff39588f98ae68b5ee9
Reviewed-on: https://go-review.googlesource.com/c/go/+/334151
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

9 files changed, 87 insertions, 40 deletions

diff --git a/src/cmd/compile/internal/types2/testdata/check/issues.go2 b/src/cmd/compile/internal/types2/testdata/check/issues.go2
index 88ae294d7c..32c4320d27 100644
--- a/src/cmd/compile/internal/types2/testdata/check/issues.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/issues.go2
@@ -232,7 +232,7 @@ 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
diff --git a/src/cmd/compile/internal/types2/testdata/check/tinference.go2 b/src/cmd/compile/internal/types2/testdata/check/tinference.go2
index 2fdb39ca7a..1b70981759 100644
--- a/src/cmd/compile/internal/types2/testdata/check/tinference.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/tinference.go2
@@ -8,19 +8,20 @@ import "strconv"
 
 type any interface{}
 
-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 _() {
@@ -29,13 +30,14 @@ 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 _() {
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39634.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39634.go2
index 6d002f5d2f..5cb15e7e58 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39634.go2
+++ b/src/cmd/compile/internal/types2/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] }
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39680.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39680.go2
index 01eadd2dbf..e56bc35475 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39680.go2
+++ b/src/cmd/compile/internal/types2/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"})
 }
+*/
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39948.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39948.go2
index 6372397ed9..e38e57268d 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue39948.go2
+++ b/src/cmd/compile/internal/types2/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(gri) 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
 }
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47127.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue47127.go2
new file mode 100644
index 0000000000..387c946957
--- /dev/null
+++ b/src/cmd/compile/internal/types2/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" */ }]()
diff --git a/src/cmd/compile/internal/types2/typeset.go b/src/cmd/compile/internal/types2/typeset.go
index 4aee8e4097..8e6af8e65c 100644
--- a/src/cmd/compile/internal/types2/typeset.go
+++ b/src/cmd/compile/internal/types2/typeset.go
@@ -216,11 +216,9 @@ func computeTypeSet(check *Checker, pos syntax.Pos, ityp *Interface) *TypeSet {
 			//           interface before go1.18.
 			types = typ
 		case *TypeParam:
-			if check != nil && !check.allowVersion(check.pkg, 1, 18) {
-				check.errorf(pos, "%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
diff --git a/src/cmd/compile/internal/types2/union.go b/src/cmd/compile/internal/types2/union.go
index e5e851c1d2..5983a73ec6 100644
--- a/src/cmd/compile/internal/types2/union.go
+++ b/src/cmd/compile/internal/types2/union.go
@@ -128,13 +128,18 @@ func parseUnion(check *Checker, tlist []syntax.Expr) Type {
 	return newUnion(types, tilde)
 }
 
-func parseTilde(check *Checker, x syntax.Expr) (Type, bool) {
-	tilde := false
+func parseTilde(check *Checker, x syntax.Expr) (typ Type, tilde bool) {
 	if op, _ := x.(*syntax.Operation); op != nil && op.Op == syntax.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, "cannot embed a type parameter")
+		typ = Typ[Invalid]
+	}
+	return
 }
 
 // intersect computes the intersection of the types x and y,
diff --git a/test/typeparam/typelist.go b/test/typeparam/typelist.go
index bd90d86fcf..5c51c9c461 100644
--- a/test/typeparam/typelist.go
+++ b/test/typeparam/typelist.go
@@ -67,6 +67,8 @@ func _[V any, T interface { type map[string]V }](p T) V {
 // Testing partial and full type inference, including the case where the types can
 // be inferred without needing the types of the function arguments.
 
+// Cannot embed stand-alone type parameters. Disabled for now.
+/*
 func f0[A any, B interface{type C}, C interface{type D}, D interface{type A}](A, B, C, D)
 func _() {
         f := f0[string]
@@ -82,6 +84,7 @@ func _() {
         f(0, 0)
         f1(0, 0)
 }
+*/
 
 func f2[A any, B interface{type []A}](_ A, _ B)
 func _() {
@@ -92,6 +95,8 @@ func _() {
 	// f2(0, []byte{}) - this one doesn't work
 }
 
+// Cannot embed stand-alone type parameters. Disabled for now.
+/*
 func f3[A any, B interface{type C}, C interface{type *A}](a A, _ B, c C)
 func _() {
 	f := f3[int]
@@ -99,6 +104,7 @@ func _() {
 	f(x, &x, &x)
 	f3(x, &x, &x)
 }
+*/
 
 func f4[A any, B interface{type []C}, C interface{type *A}](_ A, _ B, c C)
 func _() {