diff --git a/README.md b/README.md
index dd40471feaae3..72ff037c9af10 100644
--- a/README.md
+++ b/README.md
@@ -167,3 +167,4 @@ view of the available documentation. In particular, the documents titled
 [Debugging the Swift Compiler](docs/DebuggingTheCompiler.md) and
 [Continuous Integration for Swift](docs/ContinuousIntegration.md) are very
 helpful to understand before submitting your first PR.
+
diff --git a/lib/SILGen/SILGenPoly.cpp b/lib/SILGen/SILGenPoly.cpp
index 176b50f8df41f..09d95256b66fe 100644
--- a/lib/SILGen/SILGenPoly.cpp
+++ b/lib/SILGen/SILGenPoly.cpp
@@ -6347,13 +6347,10 @@ SILFunction *SILGenModule::getOrCreateCustomDerivativeThunk(
     arguments.push_back(indErrorRes.getLValueAddress());
   forwardFunctionArguments(thunkSGF, loc, fnRefType, params, arguments);
 
-  SubstitutionMap subs = thunk->getForwardingSubstitutionMap();
-  SILType substFnType = fnRef->getType().substGenericArgs(
-      M, subs, thunk->getTypeExpansionContext());
-
   // Apply function argument.
-  auto apply =
-      thunkSGF.emitApplyWithRethrow(loc, fnRef, substFnType, subs, arguments);
+  auto apply = thunkSGF.emitApplyWithRethrow(
+      loc, fnRef, /*substFnType*/ fnRef->getType(),
+      thunk->getForwardingSubstitutionMap(), arguments);
 
   // Self reordering thunk is necessary if wrt at least two parameters,
   // including self.
diff --git a/lib/SILOptimizer/Mandatory/Differentiation.cpp b/lib/SILOptimizer/Mandatory/Differentiation.cpp
index 3584fba568a3c..b8251eccfdd09 100644
--- a/lib/SILOptimizer/Mandatory/Differentiation.cpp
+++ b/lib/SILOptimizer/Mandatory/Differentiation.cpp
@@ -906,15 +906,6 @@ bool DifferentiationTransformer::canonicalizeDifferentiabilityWitness(
       traceMessage.c_str(), witness->getOriginalFunction());
 
   assert(witness->isDefinition());
-  SILFunction *orig = witness->getOriginalFunction();
-
-  // We can generate empty JVP / VJP for functions available externally. These
-  // functions have the same linkage as the original ones sans `external`
-  // flag. Important exception here hidden_external functions as they are
-  // serializable but corresponding hidden ones would be not and the SIL
-  // verifier will fail. Patch `serializeFunctions` for this case.
-  if (orig->getLinkage() == SILLinkage::HiddenExternal)
-    serializeFunctions = IsNotSerialized;
 
   // If the JVP doesn't exist, need to synthesize it.
   if (!witness->getJVP()) {
@@ -923,8 +914,9 @@ bool DifferentiationTransformer::canonicalizeDifferentiabilityWitness(
     // - Functions with unsupported control flow.
     if (context.getASTContext()
             .LangOpts.hasFeature(Feature::ForwardModeDifferentiation) &&
-        (diagnoseNoReturn(context, orig, invoker) ||
-         diagnoseUnsupportedControlFlow(context, orig, invoker)))
+        (diagnoseNoReturn(context, witness->getOriginalFunction(), invoker) ||
+         diagnoseUnsupportedControlFlow(
+             context, witness->getOriginalFunction(), invoker)))
       return true;
 
     // Create empty JVP.
@@ -941,10 +933,10 @@ bool DifferentiationTransformer::canonicalizeDifferentiabilityWitness(
         !witness->getVJP()) {
       // JVP and differential generation do not currently support functions with
       // multiple basic blocks.
-      if (orig->size() > 1) {
-        context.emitNondifferentiabilityError(orig->getLocation().getSourceLoc(),
-                                              invoker,
-                                              diag::autodiff_jvp_control_flow_not_supported);
+      if (witness->getOriginalFunction()->size() > 1) {
+        context.emitNondifferentiabilityError(
+            witness->getOriginalFunction()->getLocation().getSourceLoc(),
+            invoker, diag::autodiff_jvp_control_flow_not_supported);
         return true;
       }
       // Emit JVP function.
@@ -958,7 +950,7 @@ bool DifferentiationTransformer::canonicalizeDifferentiabilityWitness(
                      "_fatalErrorForwardModeDifferentiationDisabled");
       LLVM_DEBUG(getADDebugStream()
                  << "Generated empty JVP for "
-                 << orig->getName() << ":\n"
+                 << witness->getOriginalFunction()->getName() << ":\n"
                  << *jvp);
     }
   }
@@ -968,8 +960,9 @@ bool DifferentiationTransformer::canonicalizeDifferentiabilityWitness(
     // Diagnose:
     // - Functions with no return.
     // - Functions with unsupported control flow.
-    if (diagnoseNoReturn(context, orig, invoker) ||
-        diagnoseUnsupportedControlFlow(context, orig, invoker))
+    if (diagnoseNoReturn(context, witness->getOriginalFunction(), invoker) ||
+        diagnoseUnsupportedControlFlow(
+            context, witness->getOriginalFunction(), invoker))
       return true;
 
     // Create empty VJP.
diff --git a/test/AutoDiff/SILGen/nil_coalescing.swift b/test/AutoDiff/SILGen/nil_coalescing.swift
deleted file mode 100644
index 994fbeed691fa..0000000000000
--- a/test/AutoDiff/SILGen/nil_coalescing.swift
+++ /dev/null
@@ -1,25 +0,0 @@
-// RUN: %target-swift-frontend -emit-sil -verify %s | %FileCheck %s
-
-import _Differentiation
-
-// CHECK: sil @test_nil_coalescing
-// CHECK: bb0(%{{.*}} : $*T, %[[ARG_OPT:.*]] : $*Optional<T>, %[[ARG_PB:.*]] :
-// CHECK:    $@noescape @callee_guaranteed @substituted <τ_0_0> () -> (@out τ_0_0, @error any Error) for <T>):
-// CHECK: %[[ALLOC_OPT:.*]] = alloc_stack [lexical] $Optional<T>
-// CHECK: copy_addr %[[ARG_OPT]] to [init] %[[ALLOC_OPT]] : $*Optional<T>
-// CHECK: switch_enum_addr %[[ALLOC_OPT]] : $*Optional<T>, case #Optional.some!enumelt: {{.*}}, case #Optional.none!enumelt: {{.*}}
-// CHECK: try_apply %[[ARG_PB]](%{{.*}}) : $@noescape @callee_guaranteed @substituted <τ_0_0> () -> (@out τ_0_0, @error any Error) for <T>, normal {{.*}}, error {{.*}}
-//
-@_silgen_name("test_nil_coalescing")
-@derivative(of: ??)
-@usableFromInline
-func nilCoalescing<T: Differentiable>(optional: T?, defaultValue: @autoclosure () throws -> T)
- rethrows -> (value: T, pullback: (T.TangentVector) -> Optional<T>.TangentVector)
-{
- let hasValue = optional != nil
- let value = try optional ?? defaultValue()
- func pullback(_ v: T.TangentVector) -> Optional<T>.TangentVector {
-   return hasValue ? .init(v) : .zero
- }
- return (value, pullback)
-}