Diagnose invalid Swift-only @objcImpl inits

Required and designated inits have to be overridable—the former so that uses of the initializer on `any T.Type` will call the subclass initializer, the latter so that inherited convenience inits will call the subclass initializer. However, Swift-only members use vtables to dispatch to subclasses, and @objcImpl classes don’t have vtables, so their Swift-only inits cannot be made overridable. Upshot: Swift-only inits on @objcImpl classes must be `convenience`, not `required` or designated.

Enforce this rule in the ObjCImplementationChecker.

Fixes rdar://109121293.

Author: beccadax

include/swift/AST/DiagnosticsSema.def

@@ -1841,6 +1841,20 @@ NOTE(objc_implementation_requirement_here,none,
      "%kind0 declared in header here",
      (ValueDecl *))
 
+ERROR(objc_implementation_init_must_be_convenience, none,
+      "%kind0 is not valid in an '@_objcImplementation' extension because "
+      "Objective-C subclasses must be able to override "
+      "%select{designated|required}1 initializers",
+      (const ValueDecl *, /*isRequired=*/bool))
+NOTE(objc_implementation_init_turn_designated_to_convenience, none,
+     "add 'convenience' keyword to make this a convenience initializer",
+     ())
+NOTE(objc_implementation_init_turn_required_to_convenience, none,
+     "replace 'required' keyword with 'convenience' to make this a convenience "
+     "initializer",
+     ())
+
+// Fallback diagnostic; super-general by nature.
 ERROR(objc_implementation_member_requires_vtable, none,
       "%kind0 is not valid in an '@_objcImplementation' extension because it "
       "is an overridable Swift-only %kindonly0",

lib/Sema/TypeCheckDeclObjC.cpp

@@ -2984,6 +2984,28 @@ class ObjCImplementationChecker {
     if (attr->isInvalid())
       return;
 
+    if (auto init = dyn_cast<ConstructorDecl>(afd)) {
+      if (!init->isObjC() && (init->isRequired() ||
+                              !init->isConvenienceInit())) {
+        // Swift-only initializers have to be convenience.
+        diagnose(afd, diag::objc_implementation_init_must_be_convenience,
+                 afd, init->isRequired());
+
+        // Add appropriate fix-it to 'convenience'.
+        if (auto requiredMod = init->getAttrs().getAttribute<RequiredAttr>())
+          diagnose(afd,
+                   diag::objc_implementation_init_turn_required_to_convenience)
+            .fixItReplace(requiredMod->getRange(), "convenience");
+        else
+          diagnose(afd,
+                   diag::objc_implementation_init_turn_designated_to_convenience)
+            .fixItInsert(afd->getAttributeInsertionLoc(/*forModifier=*/true),
+                         "convenience ");
+
+        return;
+      }
+    }
+
     // Emit a vague fallback diagnostic.
     diagnose(afd, diag::objc_implementation_member_requires_vtable, afd);
   }

test/decl/ext/objc_implementation.swift

@@ -203,11 +203,13 @@ protocol EmptySwiftProto {}
   }
 
   @nonobjc public init(notFromHeader4: CInt) {
-    // expected-warning@-1 {{initializer 'init(notFromHeader4:)' is not valid in an '@_objcImplementation' extension because it is an overridable Swift-only initializer}}
+    // expected-warning@-1 {{initializer 'init(notFromHeader4:)' is not valid in an '@_objcImplementation' extension because Objective-C subclasses must be able to override designated initializers}}
+    // expected-note@-2 {{add 'convenience' keyword to make this a convenience initializer}} {{12-12=convenience }}
   }
 
   @nonobjc public required init(notFromHeader5: CInt) {
-    // expected-warning@-1 {{initializer 'init(notFromHeader5:)' is not valid in an '@_objcImplementation' extension because it is an overridable Swift-only initializer}}
+    // expected-warning@-1 {{initializer 'init(notFromHeader5:)' is not valid in an '@_objcImplementation' extension because Objective-C subclasses must be able to override required initializers}}
+    // expected-note@-2 {{replace 'required' keyword with 'convenience' to make this a convenience initializer}} {{19-27=convenience}}
   }
 
   @nonobjc public convenience init(notFromHeader6: CInt) {