Fixes #59100

The above referenced issue was causing a compiler crash when writing
differentiable protocols and corresponding implementers, without importing
the `_Differentiation` module.

Changes in this CR fix the issue by marking any `@differentiable`
attributes as invalid, if the `_Differentiation` module has not been
imported. This ignores the `@differentiable` attributes when the
protocol witnesses are being verified. Witness verification was previously
leading to an error (due to missing `@differentiable` attribute on the protocol
requirement implementer), and the corresponding diagnostic emission code was
then leading to a crash, because it was expecting the `_Differentiation`
module to be present.

Author: jkshtj

lib/Sema/TypeCheckAttr.cpp

@@ -5780,8 +5780,10 @@ IndexSubset *DifferentiableAttributeTypeCheckRequest::evaluate(
 
   // `@differentiable` attribute requires experimental differentiable
   // programming to be enabled.
-  if (checkIfDifferentiableProgrammingEnabled(attr, D))
+  if (checkIfDifferentiableProgrammingEnabled(attr, D)) {
+    attr->setInvalid();
     return nullptr;
+  }
 
   // If `@differentiable` attribute is declared directly on a
   // `AbstractStorageDecl` (a stored/computed property or subscript),
@@ -5791,8 +5793,10 @@ IndexSubset *DifferentiableAttributeTypeCheckRequest::evaluate(
 
   // Resolve the original `AbstractFunctionDecl`.
   auto *original = resolveDifferentiableAttrOriginalFunction(attr);
-  if (!original)
+  if (!original) {
+    attr->setInvalid();
     return nullptr;
+  }
 
   return typecheckDifferentiableAttrforDecl(original, attr);
 }

test/AutoDiff/Sema/differentiable_swift_without_import.swift

@@ -0,0 +1,16 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P {
+  @differentiable(reverse) // expected-error {{@differentiable attribute used without importing module '_Differentiation'}}
+  func req()
+}
+
+struct S: P {
+  // Had the `_Differentiation` module been imported, this conformance 
+  // requirement would have generated an error, complaining about the 
+  // missing `@derivative` attribute. However, because the `_Differentiation`
+  // module import is missing, the `@differential` attribute on `P` is marked
+  // invalid and therefore not used while validating witnesses (specifically `S.req()`) 
+  // for the `P.req()` requirement.
+  public func req() {}
+}