Fix bug and add test case.

Author: bartchr808

lib/SILOptimizer/Mandatory/Differentiation.cpp

@@ -3235,7 +3235,8 @@ class VJPEmitter final
     // This instruction is active. Determine the appropriate differentiation
     // strategy, and use it.
     auto *structDecl = sei->getStructDecl();
-    if (structDecl->getAttrs().hasAttribute<FieldwiseDifferentiableAttr>()) {
+    if (structDecl->getEffectiveAccess() <= AccessLevel::Internal
+        || structDecl->getAttrs().hasAttribute<FieldwiseDifferentiableAttr>()) {
       strategies[sei] = StructExtractDifferentiationStrategy::Fieldwise;
       SILClonerWithScopes::visitStructExtractInst(sei);
       return;

test/AutoDiff/generics.swift

@@ -128,4 +128,16 @@ func TF_508_func(x: TF_508_Struct<Float>, y: TF_508_Struct<Float>)
 }
 let TF_508_bp = pullback(at: TF_508_inst, TF_508_inst, in: TF_508_func)
 
+// TF-523
+struct A : Differentiable & AdditiveArithmetic {
+  var a: Float = 1
+  typealias TangentVector = A
+  typealias AllDifferentiableVariables = A
+}
+
+@differentiable
+func f(_ x: A) -> Float {
+  return x.a * 2
+}
+
 // TODO: add more tests.