Gate @derivative attribute by differentiable programming flag.

Gate `@derivative` attribute by
`-enable-experimental-differentiable-programming` flag.

Author: dan-zheng

lib/Sema/TypeCheckAttr.cpp

@@ -3354,7 +3354,15 @@ getAutoDiffOriginalFunctionType(AnyFunctionType *derivativeFnTy) {
 }
 
 void AttributeChecker::visitDerivativeAttr(DerivativeAttr *attr) {
-  FuncDecl *derivative = cast<FuncDecl>(D);
+  // `@derivative` attribute requires experimental differentiable programming
+  // to be enabled.
+  auto &ctx = D->getASTContext();
+  if (!ctx.LangOpts.EnableExperimentalDifferentiableProgramming) {
+    diagnoseAndRemoveAttr(
+        attr, diag::experimental_differentiable_programming_disabled);
+    return;
+  }
+  auto *derivative = cast<FuncDecl>(D);
   auto lookupConformance =
       LookUpConformanceInModule(D->getDeclContext()->getParentModule());
   auto originalName = attr->getOriginalFunctionName();

test/AutoDiff/Sema/differentiable_features_disabled.swift

@@ -2,3 +2,12 @@
 
 // expected-error @+1 {{differentiable programming is an experimental feature that is currently disabled}}
 let _: @differentiable (Float) -> Float
+
+func id(_ x: Float) -> Float {
+  return x
+}
+// expected-error @+1 {{differentiable programming is an experimental feature that is currently disabled}}
+@derivative(of: id)
+func jvpId(x: Float) -> (value: Float, differential: (Float) -> (Float)) {
+  return (x, { $0 })
+}