Diagnose lightweight generic classes with objcImpl

Extensions to lightweight generic classes are such a huge mess that we’re just removing these from the feature’s scope for now.

Fixes rdar://116066409.

Author: beccadax

include/swift/AST/DiagnosticsSema.def

@@ -1722,6 +1722,9 @@ ERROR(attr_objc_implementation_must_have_super,none,
       "'@_objcImplementation' cannot be used to implement root %kind0; declare "
       "its superclass in the header",
       (ValueDecl *))
+ERROR(objc_implementation_cannot_have_generics,none,
+      "'@_objcImplementation' cannot be used to implement %kind0",
+      (ValueDecl *))
 ERROR(attr_objc_implementation_category_not_found,none,
       "could not find category %0 on Objective-C class %1; make sure your "
       "umbrella or bridging header imports the header that declares it",

lib/Sema/TypeCheckAttr.cpp

@@ -1510,6 +1510,12 @@ visitObjCImplementationAttr(ObjCImplementationAttr *attr) {
     return;
   }
 
+  if (CD->isTypeErasedGenericClass()) {
+    diagnoseAndRemoveAttr(attr, diag::objc_implementation_cannot_have_generics,
+                          CD);
+    CD->diagnose(diag::decl_declared_here, CD);
+  }
+
   if (!attr->isCategoryNameInvalid() && !ED->getImplementedObjCDecl()) {
     diagnose(attr->getLocation(),
              diag::attr_objc_implementation_category_not_found,

test/decl/ext/Inputs/objc_implementation.h

@@ -172,6 +172,10 @@
 
 @end
 
+@interface ObjCImplGenericClass<T> : NSObject
+
+@end
+
 
 
 struct ObjCStruct {

test/decl/ext/objc_implementation.swift

@@ -460,6 +460,10 @@ protocol EmptySwiftProto {}
   // expected-error@-1 {{'@_objcImplementation' cannot be used to implement root class 'ObjCImplRootClass'; declare its superclass in the header}}
 }
 
+@_objcImplementation extension ObjCImplGenericClass {
+  // expected-error@-1 {{'@_objcImplementation' cannot be used to implement generic class 'ObjCImplGenericClass'}}
+}
+
 func usesAreNotAmbiguous(obj: ObjCClass) {
   obj.method(fromHeader1: 1)
   obj.method(fromHeader2: 2)