[Sema] Reject an @objc generic subscript

Previously we could allow such a declaration to be
marked @objc, despite being incompatible. This
caused crashes later down the pipeline as the
subscript's accessors were correctly checked for
generic params and were not marked @objc.

Resolves SR-12801.

Author: hamishknight

include/swift/AST/DiagnosticsSema.def

@@ -4239,8 +4239,8 @@ ERROR(objc_invalid_on_subscript,none,
 ERROR(objc_invalid_on_static_subscript,none,
       "%0 cannot be %" OBJC_ATTR_SELECT "1", (DescriptiveDeclKind, unsigned))
 ERROR(objc_invalid_with_generic_params,none,
-      "method cannot be %" OBJC_ATTR_SELECT "0 because it has generic "
-      "parameters", (unsigned))
+      "%0 cannot be %" OBJC_ATTR_SELECT "1 because it has generic parameters",
+      (DescriptiveDeclKind, unsigned))
 ERROR(objc_convention_invalid,none,
       "%0 is not representable in Objective-C, so it cannot be used"
       " with '@convention(%1)'", (Type, StringRef))

lib/Sema/TypeCheckDeclObjC.cpp

@@ -304,16 +304,17 @@ static bool isParamListRepresentableInObjC(const AbstractFunctionDecl *AFD,
 
 /// Check whether the given declaration contains its own generic parameters,
 /// and therefore is not representable in Objective-C.
-static bool checkObjCWithGenericParams(const AbstractFunctionDecl *AFD,
-                                       ObjCReason Reason) {
-  bool Diagnose = shouldDiagnoseObjCReason(Reason, AFD->getASTContext());
+static bool checkObjCWithGenericParams(const ValueDecl *VD, ObjCReason Reason) {
+  bool Diagnose = shouldDiagnoseObjCReason(Reason, VD->getASTContext());
 
-  if (AFD->getGenericParams()) {
+  auto *GC = VD->getAsGenericContext();
+  assert(GC);
+  if (GC->getGenericParams()) {
     // Diagnose this problem, if asked to.
     if (Diagnose) {
-      AFD->diagnose(diag::objc_invalid_with_generic_params,
-                    getObjCDiagnosticAttrKind(Reason));
-      describeObjCReason(AFD, Reason);
+      VD->diagnose(diag::objc_invalid_with_generic_params,
+                   VD->getDescriptiveKind(), getObjCDiagnosticAttrKind(Reason));
+      describeObjCReason(VD, Reason);
     }
 
     return true;
@@ -855,6 +856,8 @@ bool swift::isRepresentableInObjC(const SubscriptDecl *SD, ObjCReason Reason) {
 
   if (checkObjCInForeignClassContext(SD, Reason))
     return false;
+  if (checkObjCWithGenericParams(SD, Reason))
+    return false;
 
   // ObjC doesn't support class subscripts.
   if (!SD->isInstanceMember()) {

test/attr/attr_objc.swift

@@ -2375,3 +2375,9 @@ class SR_9035_C {}
   func throwingMethod2() throws -> Unmanaged<SR_9035_C> // expected-error {{method cannot be a member of an @objc protocol because its result type cannot be represented in Objective-C}}
   // expected-note@-1 {{inferring '@objc' because the declaration is a member of an '@objc' protocol}}
 }
+
+// SR-12801: Make sure we reject an @objc generic subscript.
+class SR12801 {
+  @objc subscript<T>(foo : [T]) -> Int { return 0 }
+  // expected-error@-1 {{subscript cannot be marked @objc because it has generic parameters}}
+}