Soften objcImpl implicit final errors to warnings

Before the change from @_objcImplementation to @objc @implementation, if a member was unrepresentable in ObjC, it would become implicitly `final`. After that change, this is now an error. We do want a diagnostic here, but we don’t want to break backwards compatibility for early adopters.

Soften the error to a warning when the old @objcImplementation syntax is used.

Fixes rdar://129247349.

Author: beccadax

lib/Sema/TypeCheckDeclObjC.cpp

@@ -38,6 +38,13 @@ using namespace swift;
 DiagnosticBehavior
 swift::behaviorLimitForObjCReason(ObjCReason reason, ASTContext &ctx) {
   switch(reason) {
+  case ObjCReason::MemberOfObjCImplementationExtension:
+    // If they're using the old syntax, soften to a warning.
+    if (cast<ObjCImplementationAttr>(reason.getAttr())->isEarlyAdopter())
+      return DiagnosticBehavior::Warning;
+
+    LLVM_FALLTHROUGH;
+
   case ObjCReason::ExplicitlyCDecl:
   case ObjCReason::ExplicitlyDynamic:
   case ObjCReason::ExplicitlyObjC:
@@ -51,7 +58,6 @@ swift::behaviorLimitForObjCReason(ObjCReason reason, ASTContext &ctx) {
   case ObjCReason::WitnessToObjC:
   case ObjCReason::ImplicitlyObjC:
   case ObjCReason::MemberOfObjCExtension:
-  case ObjCReason::MemberOfObjCImplementationExtension:
     return DiagnosticBehavior::Unspecified;
 
   case ObjCReason::ExplicitlyIBInspectable:

test/decl/ext/objc_implementation.swift

@@ -232,6 +232,11 @@ protocol EmptySwiftProto {}
   // rdar://122280735 - crash when the parameter of a block property needs @escaping
   let rdar122280735: (() -> ()) -> Void = { _ in }
   // expected-error@-1 {{property 'rdar122280735' of type '(() -> ()) -> Void' does not match type '(@escaping () -> Void) -> Void' declared by the header}}
+
+  private func privateNonObjCMethod(_: EmptySwiftProto) {
+    // expected-error@-1 {{method cannot be in an @objc @implementation extension of a class (without final or @nonobjc) because the type of the parameter cannot be represented in Objective-C}}
+    // expected-note@-2 {{protocol-constrained type containing protocol 'EmptySwiftProto' cannot be represented in Objective-C}}
+  }
 }
 
 @objc(PresentAdditions) @implementation extension ObjCClass {

test/decl/ext/objc_implementation_early_adopter.swift

@@ -229,6 +229,11 @@ protocol EmptySwiftProto {}
   // rdar://122280735 - crash when the parameter of a block property needs @escaping
   let rdar122280735: (() -> ()) -> Void = { _ in }
   // expected-warning@-1 {{property 'rdar122280735' of type '(() -> ()) -> Void' does not match type '(@escaping () -> Void) -> Void' declared by the header}}
+
+  private func privateNonObjCMethod(_: EmptySwiftProto) {
+    // expected-warning@-1 {{method cannot be in an @objc @implementation extension of a class (without final or @nonobjc) because the type of the parameter cannot be represented in Objective-C}}
+    // expected-note@-2 {{protocol-constrained type containing protocol 'EmptySwiftProto' cannot be represented in Objective-C}}
+  }
 }
 
 @_objcImplementation(PresentAdditions) extension ObjCClass {
@@ -428,8 +433,8 @@ protocol EmptySwiftProto {}
   func nullableResult() -> Any { fatalError() } // expected-warning {{instance method 'nullableResult()' of type '() -> Any' does not match type '() -> Any?' declared by the header}}
   func nullableArgument(_: Any) {} // expected-warning {{instance method 'nullableArgument' of type '(Any) -> ()' does not match type '(Any?) -> Void' declared by the header}}
 
-  func nonPointerResult() -> CInt! { fatalError() } // expected-error{{method cannot be in an @objc @implementation extension of a class (without final or @nonobjc) because its result type cannot be represented in Objective-C}}
-  func nonPointerArgument(_: CInt!) {} // expected-error {{method cannot be in an @objc @implementation extension of a class (without final or @nonobjc) because the type of the parameter cannot be represented in Objective-C}}
+  func nonPointerResult() -> CInt! { fatalError() } // expected-warning{{method cannot be in an @objc @implementation extension of a class (without final or @nonobjc) because its result type cannot be represented in Objective-C}}
+  func nonPointerArgument(_: CInt!) {} // expected-warning {{method cannot be in an @objc @implementation extension of a class (without final or @nonobjc) because the type of the parameter cannot be represented in Objective-C}}
 }
 
 // Intentionally using `@_objcImplementation` for this test; do not upgrade!