Sema: ban extensions of invertible protocols

There's no good reason to permit them. Unlike Sendable, Copyable
conformances are pervasive, so this is as if we're permitting
extensions of `Any`. Until there's a good argument in favor of such
extensions, I'm removing the capability now.

Author: kavon

include/swift/AST/DiagnosticsSema.def

@@ -2537,8 +2537,8 @@ ERROR(invalid_nominal_extension,none,
       (Type, Type))
 NOTE(invalid_extension_rewrite,none,
      "did you mean to extend %0 instead?", (Type))
-ERROR(synthesized_nominal_extension,none,
-      "cannot extend synthesized type %0", (Type))
+ERROR(cannot_extend_nominal,none,
+      "cannot extend %kind0", (const NominalTypeDecl *))
 
 ERROR(retroactive_not_in_extension_inheritance_clause,none,
       "'retroactive' attribute only applies in inheritance clauses in "

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -3676,6 +3676,15 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     return true;
   }
 
+  static void diagnoseExtensionOfInvertible(ExtensionDecl *ED) {
+    auto *nominal = ED->getExtendedNominal();
+    if (auto *proto = dyn_cast_or_null<ProtocolDecl>(nominal)) {
+      if (proto->getInvertibleProtocolKind()) {
+        ED->diagnose(diag::cannot_extend_nominal, nominal);
+      }
+    }
+  }
+
   static void checkTupleExtension(ExtensionDecl *ED) {
     auto *nominal = ED->getExtendedNominal();
     if (!nominal || !isa<BuiltinTupleDecl>(nominal))
@@ -3878,6 +3887,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       TypeChecker::checkDistributedActor(SF, nominal);
 
     diagnoseIncompatibleProtocolsForMoveOnlyType(ED);
+    diagnoseExtensionOfInvertible(ED);
 
     checkTupleExtension(ED);
   }

test/Sema/copyable.swift

@@ -26,3 +26,7 @@ enum namespace {
 
   func Copyable() -> Copyable { return 0 }
 }
+
+extension Copyable { // expected-error {{cannot extend protocol 'Copyable'}}
+  func hello() {}
+}