Revert "[AutoDiff] Fix custom derivative thunk for Optional (#71721)" (#74268)

This reverts commit aa5dddb.

Fixes `preset=buildbot,tools=RA,stdlib=DA` CI job, which without this revert fails on `AutoDiff/SILGen/nil_coalescing.swift` test.

Author: MaxDesiatov

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.

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.