Make stored props/vars not have custom VJPs/JVPs.

Author: bartchr808

include/swift/AST/DiagnosticsSema.def

@@ -2733,8 +2733,8 @@ ERROR(differentiable_attr_unsupported_req_kind,none,
       "layout requirement are not supported by '@differentiable' attribute", ())
 ERROR(differentiable_attr_class_unsupported,none,
       "class members cannot be marked with '@differentiable'", ())
-ERROR(differentiable_attr_stored_property_unsupported,none,
-      "stored properties cannot be marked with '@differentiable'", ())
+ERROR(differentiable_attr_stored_property_variable_unsupported,none,
+      "stored properties/variables cannot be marked with '@differentiable' with a custom VJP/JVP", ())
 NOTE(protocol_witness_missing_specific_differentiable_attr,none,
      "candidate is missing attribute '%0'", (StringRef))
 

lib/Sema/TypeCheckAttr.cpp

@@ -2887,8 +2887,10 @@ void AttributeChecker::visitDifferentiableAttr(DifferentiableAttr *attr) {
 
   AbstractFunctionDecl *original = dyn_cast<AbstractFunctionDecl>(D);
   if (auto *asd = dyn_cast<AbstractStorageDecl>(D)) {
-    if (asd->getImplInfo().isSimpleStored()) {
-      diagnoseAndRemoveAttr(attr, diag::differentiable_attr_stored_property_unsupported);
+    if (asd->getImplInfo().isSimpleStored() &&
+        (attr->getJVP() || attr->getVJP())) {
+      diagnoseAndRemoveAttr(attr,
+          diag::differentiable_attr_stored_property_variable_unsupported);
 	  return;
     }
     // When used directly on a storage decl (stored/computed property or