[Sema] Diagnose invalid uses of protocol suppression via ~ syntax

include/swift/AST/DiagnosticsSema.def

@@ -8950,6 +8950,10 @@ ERROR(tilde_sendable_requires_feature_flag,none,
       "'~Sendable' requires -enable-experimental-feature TildeSendable",
       ())
 
+ERROR(conformance_repression_only_on_struct_class_enum,none,
+      "conformance to %0 can only be suppressed on structs, classes, and enums",
+      (const ProtocolDecl *))
+
 //===----------------------------------------------------------------------===//
 // MARK: Swift Performance hints
 //===----------------------------------------------------------------------===//

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -132,6 +132,18 @@ public:
       }
     }
 
+    if (auto *TD = dyn_cast<const TypeDecl *>(decl)) {
+      if (!(isa<StructDecl>(TD) || isa<ClassDecl>(TD) || isa<EnumDecl>(TD))) {
+        diagnoseInvalid(repr, repr.getLoc(),
+                        diag::conformance_repression_only_on_struct_class_enum,
+                        ctx.getProtocol(*kp))
+            // Downgrade to a warning for `~BitwiseCopyable` because it was accepted
+            // in some incorrect positions before.
+            .warnUntilFutureSwiftVersionIf(kp == KnownProtocolKind::BitwiseCopyable);
+        return Type();
+      }
+    }
+
     if (auto *extension = dyn_cast<const ExtensionDecl *>(decl)) {
       diagnoseInvalid(repr, extension,
                       diag::suppress_inferrable_protocol_extension,

test/Sema/bitwise_copyable_2.swift

@@ -7,6 +7,23 @@
 
 // This test file only exists in order to test without noncopyable_generics and can be deleted once that is always enabled.
 
+protocol P: ~BitwiseCopyable {
+  // expected-warning@-1 {{conformance to 'BitwiseCopyable' can only be suppressed on structs, classes, and enums; this will be an error in a future}}
+}
+
+protocol Q {
+  associatedtype V: ~BitwiseCopyable
+  // expected-warning@-1 {{conformance to 'BitwiseCopyable' can only be suppressed on structs, classes, and enums; this will be an error in a future}}
+}
+
+func test<T: ~BitwiseCopyable>(_: T) {
+  // expected-warning@-1 {{conformance to 'BitwiseCopyable' can only be suppressed on structs, classes, and enums; this will be an error in a future}}
+}
+
+func test<T>() -> T where T: ~BitwiseCopyable {
+  // expected-error@-1 {{type 'BitwiseCopyable' cannot be suppressed}}
+}
+
 @_nonescapable
 struct S_Implicit_Nonescapable {}
 

test/Sema/tilde_sendable.swift

@@ -0,0 +1,34 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature TildeSendable
+
+// REQUIRES: swift_feature_TildeSendable
+
+protocol P: ~Sendable { // expected-error {{conformance to 'Sendable' can only be suppressed on structs, classes, and enums}}
+}
+
+protocol Q {
+  associatedtype T: ~Sendable
+  // expected-error@-1 {{conformance to 'Sendable' can only be suppressed on structs, classes, and enums}}
+}
+
+struct A: ~Sendable {}
+class B: ~Sendable {}
+enum C: ~Sendable {}
+
+struct E1: Sendable, ~Sendable {}
+
+enum E2: ~Sendable, ~Sendable {} // expected-warning {{already suppressed conformance to 'Sendable'}}
+
+struct InExt {}
+
+extension InExt: ~Sendable { // expected-error {{conformance to inferrable protocol 'Sendable' cannot be suppressed in an extension}}
+}
+
+func test<T: ~Sendable>(_: T) {} // expected-error {{conformance to 'Sendable' can only be suppressed on structs, classes, and enums}}
+func test<Q>(other: Q) where Q: ~Sendable {} // expected-error {{type 'Sendable' cannot be suppressed}}
+
+struct Generic<T: ~Sendable> { // expected-error {{conformance to 'Sendable' can only be suppressed on structs, classes, and enums}}
+  var x: T
+}
+
+var x: some BinaryInteger & ~Sendable // expected-error {{type 'Sendable' cannot be suppressed}}
+