[AutoDiff] Fix custom derivative thunk for Optional

The patch fixes the issue #55882 and enables the nil coalescing operator
(aka `??`) for Optional type.

Author: asavonic

lib/SILGen/SILGenPoly.cpp

@@ -6204,10 +6204,13 @@ 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*/ fnRef->getType(),
-      thunk->getForwardingSubstitutionMap(), arguments);
+  auto apply =
+      thunkSGF.emitApplyWithRethrow(loc, fnRef, substFnType, subs, arguments);
 
   // Self reordering thunk is necessary if wrt at least two parameters,
   // including self.

test/AutoDiff/SILGen/nil_coalescing.swift

@@ -0,0 +1,25 @@
+// 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 $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)
+}