[AutoDiff] Fix protocol witness SILGen for @differentiable class methods. (swiftlang#32639)

During protocol witness SILGen for `@differentiable` class methods,
replace the `AutoDiffDerivativeFunctionIdentifier` generic signature
with the witness thunk substitution map's generic signature.

Resolves TF-1180: vtable SIL verification error.

Author: dan-zheng

include/swift/SIL/SILDeclRef.h

@@ -309,7 +309,7 @@ struct SILDeclRef {
   /// function.
   SILDeclRef asAutoDiffDerivativeFunction(
       AutoDiffDerivativeFunctionIdentifier *derivativeId) const {
-    assert(!derivativeFunctionIdentifier);
+    assert(derivativeId);
     SILDeclRef declRef = *this;
     declRef.derivativeFunctionIdentifier = derivativeId;
     return declRef;

lib/SILGen/SILGenPoly.cpp

@@ -4604,6 +4604,17 @@ getWitnessFunctionRef(SILGenFunction &SGF,
   }
   case WitnessDispatchKind::Class: {
     SILValue selfPtr = witnessParams.back().getValue();
+    // If `witness` is a derivative function `SILDeclRef`, replace the
+    // derivative function identifier's generic signature with the witness thunk
+    // substitution map's generic signature.
+    if (auto *derivativeId = witness.derivativeFunctionIdentifier) {
+      auto *newDerivativeId = AutoDiffDerivativeFunctionIdentifier::get(
+          derivativeId->getKind(), derivativeId->getParameterIndices(),
+          witnessSubs.getGenericSignature(), SGF.getASTContext());
+      return SGF.emitClassMethodRef(
+          loc, selfPtr, witness.asAutoDiffDerivativeFunction(newDerivativeId),
+          witnessFTy);
+    }
     return SGF.emitClassMethodRef(loc, selfPtr, witness, witnessFTy);
   }
   }

test/AutoDiff/compiler_crashers_fixed/tf1180-silgen-vtable-method-does-not-appear.swift

@@ -0,0 +1,37 @@
+// RUN: %target-swift-frontend -emit-silgen %s
+
+// TF-1180: SIL verification error regarding `@differentiable` class method
+// witnesses for `@differentiable` protocol requirements.
+
+import _Differentiation
+
+protocol Protocol {
+  @differentiable
+  func method(_ x: Float) -> Float
+}
+
+class Class: Protocol {
+  @differentiable
+  public func method(_ x: Float) -> Float { x }
+}
+
+// Original error:
+// SIL verification failed: method does not appear in the class's vtable: VerifyClassMethodVisitor(member).Seen
+// Verifying instruction:
+//      %2 = load_borrow %1 : $*Class                // users: %8, %4, %3
+// ->   %3 = class_method %2 : $Class, #Class.method!jvp.SU.<Self where Self : Protocol> : (Class) -> (Float) -> Float, $@convention(method) (Float, @guaranteed Class) -> (Float, @owned @callee_guaranteed (Float) -> Float) // user: %4
+//      %4 = apply %3(%0, %2) : $@convention(method) (Float, @guaranteed Class) -> (Float, @owned @callee_guaranteed (Float) -> Float) // user: %5
+// In function:
+// // AD__$s4main5ClassCAA8ProtocolA2aDP6methodyS2fFTW_jvp_SU
+// sil private [transparent] [thunk] [ossa] @AD__$s4main5ClassCAA8ProtocolA2aDP6methodyS2fFTW_jvp_SU : $@convention(witness_method: Protocol) (Float, @in_guaranteed Class) -> (Float, @owned @callee_guaranteed (Float) -> Float) {
+// // %0                                             // user: %4
+// // %1                                             // user: %2
+// bb0(%0 : $Float, %1 : $*Class):
+//   %2 = load_borrow %1 : $*Class                   // users: %8, %4, %3
+//   %3 = class_method %2 : $Class, #Class.method!jvp.SU.<Self where Self : Protocol> : (Class) -> (Float) -> Float, $@convention(method) (Float, @guaranteed Class) -> (Float, @owned @callee_guaranteed (Float) -> Float) // user: %4
+//   %4 = apply %3(%0, %2) : $@convention(method) (Float, @guaranteed Class) -> (Float, @owned @callee_guaranteed (Float) -> Float) // user: %5
+//   (%5, %6) = destructure_tuple %4 : $(Float, @callee_guaranteed (Float) -> Float) // users: %7, %7
+//   %7 = tuple (%5 : $Float, %6 : $@callee_guaranteed (Float) -> Float) // user: %9
+//   end_borrow %2 : $Class                          // id: %8
+//   return %7 : $(Float, @callee_guaranteed (Float) -> Float) // id: %9
+// } // end sil function 'AD__$s4main5ClassCAA8ProtocolA2aDP6methodyS2fFTW_jvp_SU'