Sema: Diagnose '@objc' protocols containing associated types

Fixes <https://bugs.swift.org/browse/SR-3850>,
<rdar://problem/32634928>.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -1749,6 +1749,10 @@ WARNING(associated_type_override_typealias,none,
         "associated type %0 is redundant with type %0 declared in inherited "
         "%1 %2", (DeclName, DescriptiveDeclKind, Type))
 
+ERROR(associated_type_objc,none,
+      "associated type %0 cannot be declared inside '@objc' protocol %1",
+      (DeclName, DeclName))
+
 ERROR(generic_param_access,none,
       "%0 %select{must be declared %select{"
       "%select{private|fileprivate|internal|%error|%error}3|private or fileprivate}4"

lib/Sema/TypeCheckDecl.cpp

@@ -4299,6 +4299,14 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
   void visitAssociatedTypeDecl(AssociatedTypeDecl *assocType) {
     if (!assocType->hasValidationStarted())
       TC.validateDecl(assocType);
+
+    auto *proto = assocType->getProtocol();
+    if (proto->isObjC()) {
+      TC.diagnose(assocType->getLoc(),
+                  diag::associated_type_objc,
+                  assocType->getName(),
+                  proto->getName());
+    }
   }
 
   void checkUnsupportedNestedType(NominalTypeDecl *NTD) {

test/decl/protocol/req/associated_type_objc.swift

@@ -0,0 +1,10 @@
+// RUN: %target-typecheck-verify-swift -disable-objc-attr-requires-foundation-module
+
+@objc protocol P {
+  associatedtype T
+  // expected-error@-1 {{associated type 'T' cannot be declared inside '@objc' protocol 'P'}}
+}
+
+extension P {
+  func takesT(_: T) {}
+}

test/decl/protocol/req/optional.swift

@@ -224,6 +224,7 @@ protocol optErrorProtocol {
   optional func foo(_ x: Int) // expected-error{{'optional' requirements are an Objective-C compatibility feature; add '@objc'}}{{3-3=@objc }}
   optional var bar: Int { get } // expected-error{{'optional' requirements are an Objective-C compatibility feature; add '@objc'}}{{3-3=@objc }}
   optional associatedtype Assoc  // expected-error{{'optional' modifier cannot be applied to this declaration}} {{3-12=}}
+  // expected-error@-1 {{associated type 'Assoc' cannot be declared inside '@objc' protocol 'optObjcAttributeProtocol'}}
 }
 
 @objc protocol optionalInitProto {