[dev.typeparams] cmd/compile/internal/types2: disallow ~T where T is a defined type or an interface

Change-Id: I35f6f43db00d56847da48320308f2fcfff924738
Reviewed-on: https://go-review.googlesource.com/c/go/+/324570
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>

src/cmd/compile/internal/types2/testdata/check/typeinst2.go2

@@ -176,12 +176,12 @@ type _ interface {
 // Interface type lists can contain any type, incl. *Named types.
 // Verify that we use the underlying type to compute the operational type.
 type MyInt int
-func add1[T interface{~MyInt}](x T) T {
+func add1[T interface{MyInt}](x T) T {
 	return x + 1
 }
 
 type MyString string
-func double[T interface{~MyInt | ~MyString}](x T) T {
+func double[T interface{MyInt|MyString}](x T) T {
 	return x + x
 }
 

src/cmd/compile/internal/types2/testdata/examples/constraints.go2

@@ -24,6 +24,15 @@ type (
 	_ interface{int|interface /* ERROR cannot use interface */ {}}
 )
 
+type (
+	// Tilde is not permitted on defined types or interfaces.
+	foo int
+	bar interface{}
+	_ interface{foo}
+	_ interface{~ /* ERROR invalid use of ~ */ foo }
+	_ interface{~ /* ERROR invalid use of ~ */ bar }
+)
+
 // Multiple embedded union elements are intersected. The order in which they
 // appear in the interface doesn't matter since intersection is a symmetric
 // operation.

src/cmd/compile/internal/types2/testdata/fixedbugs/issue39634.go2

@@ -36,8 +36,8 @@ func bar8[A foo8[A]](a A) {}
 func main8() {}
 
 // crash 9
-type foo9[A any] interface { ~/* ERROR cannot use interface */ foo9[A] }
+type foo9[A any] interface { foo9 /* ERROR illegal cycle */ [A] }
-func _() { var _ = new(foo9 /* ERROR interface contains type constraints */ [int]) }
+func _() { var _ = new(foo9 /* ERROR illegal cycle */ [int]) }
 
 // crash 12
 var u /* ERROR cycle */ , i [func /* ERROR used as value */ /* ERROR used as value */ (u, c /* ERROR undeclared */ /* ERROR undeclared */ ) {}(0, len /* ERROR must be called */ /* ERROR must be called */ )]c /* ERROR undeclared */ /* ERROR undeclared */

src/cmd/compile/internal/types2/union.go

@@ -106,17 +106,18 @@ func parseUnion(check *Checker, tlist []syntax.Expr) Type {
 			}
 
 			u := under(t)
-			if tilde[i] {
+			if tilde[i] && !Identical(u, t) {
-				// TODO(gri) enable this check once we have converted tests
+				check.errorf(x, "invalid use of ~ (underlying type of %s is %s)", t, u)
-				// if !Identical(u, t) {
+				continue // don't report another error for t
-				// 	check.errorf(x, "invalid use of ~ (underlying type of %s is %s)", t, u)
-				// }
 			}
 			if _, ok := u.(*Interface); ok {
+				// A single type with a ~ is a single-term union.
 				check.errorf(pos, "cannot use interface %s with ~ or inside a union (implementation restriction)", t)
+				continue // don't report another error for t
 			}
 
 			// Complain about duplicate entries a|a, but also a|~a, and ~a|~a.
+			// TODO(gri) We should also exclude myint|~int since myint is included in ~int.
 			if includes(types[:i], t) {
 				// TODO(gri) this currently doesn't print the ~ if present
 				check.softErrorf(pos, "duplicate term %s in union element", t)