Sema: Relax restriction on @objc conformances for lightweight generics.

It's valid to extend an ObjC lightweight generic class to conform to an ObjC protocol. Fixes rdar://problem/39550290.

Author: jckarter

lib/Sema/TypeCheckProtocol.cpp

@@ -1495,15 +1495,21 @@ checkIndividualConformance(NormalProtocolConformance *conformance,
       return conformance;
     }
     // And... even if it isn't conditional, we still don't currently support
-    // @objc protocols in extensions.
+    // @objc protocols in extensions of Swift generic classes, because there's
+    // no stable Objective-C class object to install the protocol conformance
+    // category onto.
     if (isa<ExtensionDecl>(DC)) {
       if (auto genericT = T->getGenericAncestor()) {
-        auto isSubclass = !genericT->isEqual(T);
-        auto genericTIsGeneric = (bool)genericT->getAnyGeneric()->getGenericParams();
-        TC.diagnose(ComplainLoc, diag::objc_protocol_in_generic_extension, T,
-                    ProtoType, isSubclass, genericTIsGeneric);
-        conformance->setInvalid();
-        return conformance;
+        if (!cast<ClassDecl>(genericT->getAnyNominal())
+               ->usesObjCGenericsModel()) {
+          auto isSubclass = !genericT->isEqual(T);
+          auto genericTIsGeneric = (bool)genericT->getAnyGeneric()
+                                                 ->getGenericParams();
+          TC.diagnose(ComplainLoc, diag::objc_protocol_in_generic_extension, T,
+                      ProtoType, isSubclass, genericTIsGeneric);
+          conformance->setInvalid();
+          return conformance;
+        }
       }
     }
   }

test/decl/ext/Inputs/extension-generic-objc-protocol.h

@@ -0,0 +1,19 @@
+@import Foundation;
+
+@interface OBJCGeneric<T> : NSObject
+@end
+
+@interface OBJCGenericSubclass<T, U>: OBJCGeneric<T>
+@end
+
+@interface OBJCNongenericSubclass: OBJCGenericSubclass<id, id>
+@end
+
+@protocol OBJCProtocol1
+@end
+
+@protocol OBJCProtocol2
+@end
+
+@protocol OBJCProtocol3
+@end

test/decl/ext/extension-generic-objc-protocol.swift

@@ -1,10 +1,12 @@
-// RUN: %target-swift-frontend -typecheck -verify %s
+// RUN: %target-swift-frontend -import-objc-header %S/Inputs/extension-generic-objc-protocol.h -typecheck -verify %s
 
 // REQUIRES: objc_interop
 
 import Foundation
 
 @objc protocol P {}
+@objc protocol Q {}
+@objc protocol R {}
 
 public class C1<T> {}
 extension C1: P {}
@@ -27,3 +29,32 @@ class SubInner: Outer<Int>.Inner2 {}
 
 extension SubInner: P {}
 // expected-error@-1 {{conformance of subclass of a class from generic context 'SubInner' to @objc protocol 'P' cannot be in an extension}}
+
+// Lightweight generic ObjC classes can still be extended to conform.
+
+extension OBJCGeneric: OBJCProtocol1 {}
+extension OBJCGeneric: P {}
+extension OBJCGenericSubclass: OBJCProtocol2 {}
+extension OBJCGenericSubclass: Q {}
+extension OBJCNongenericSubclass: OBJCProtocol3 {}
+extension OBJCNongenericSubclass: R {}
+
+public class SwiftSubclassOfObjCGeneric: OBJCGeneric<AnyObject> {}
+
+extension SwiftSubclassOfObjCGeneric: OBJCProtocol2 {}
+extension SwiftSubclassOfObjCGeneric: Q {}
+
+public class SwiftGenericSubclassOfObjCGeneric<T: AnyObject>
+  : OBJCGeneric<AnyObject>
+{}
+
+extension SwiftGenericSubclassOfObjCGeneric: OBJCProtocol2 {} // expected-error {{cannot be in an extension}}
+extension SwiftGenericSubclassOfObjCGeneric: Q {} // expected-error {{cannot be in an extension}}
+
+public class SwiftNongenericSubclassOfGenericSubclassOfObjCGeneric
+  : SwiftGenericSubclassOfObjCGeneric<AnyObject>
+{}
+
+extension SwiftNongenericSubclassOfGenericSubclassOfObjCGeneric: OBJCProtocol3 {} // expected-error {{cannot be in an extension}}
+extension SwiftNongenericSubclassOfGenericSubclassOfObjCGeneric: R {} // expected-error {{cannot be in an extension}}
+