[AutoDiff] Fix crash during generic curry thunk cloning.

[dan-zheng]

Create simple BasicTypeSubstCloner inheriting from TypeSubstCloner.
Use BasicTypeSubstCloner to clone generic curry thunks, remapping types.

Resolves TF-688.

---

Context:

public struct Tensor<Scalar> {
  var x: Scalar
}
extension Tensor: Differentiable where Scalar: Differentiable {
  @differentiable
  public static func id(x: Self) -> Self {
    return x
  }
}

@differentiable(wrt: x)
public func test<Scalar: Differentiable>(
  _ x: Tensor<Scalar>,
  reduction: @differentiable (Tensor<Scalar>) -> Tensor<Scalar> = Tensor.id
) -> Tensor<Scalar> {
  reduction(x)
}

$ swift tf-688.swift
SIL verification failed: generic function definition must have a generic environment: !FTy->isPolymorphic()
In function:
// AD__$s4main6TensorVAAs14DifferentiableRzlE2id1xACyxGAG_tFZTc__differentiable_curry_thunk_src_0_wrt_0
sil shared [serializable] [thunk] @AD__$s4main6TensorVAAs14DifferentiableRzlE2id1xACyxGAG_tFZTc__differentiable_curry_thunk_src_0_wrt_0 : $@convention(thin) <τ_0_0 where τ_0_0 : Differentiable> (@thin Tensor<τ_0_0>.Type) -> @owned @differentiable @callee_guaranteed (@in_guaranteed Tensor<τ_0_0>) -> @out Tensor<τ_0_0

The reproducer above crashes during the differentiation transform (ADContext::promoteToDifferentiableFunction) because curry thunks were cloned using SILFunctionCloner without accounting for generics.

With this patch, curry thunk cloning handles generics correctly by setting the new thunk's generic environment and remapping types with a TypeSubstCloner subclass.

[dan-zheng]

@swift-ci Please test tensorflow

[rxwei]

SILCloner already knows how to clone function arguments. I’ve been leaning towards getting rid of ‘createEntryArguments’ instead of using it in more places.

[rxwei]

All you need to overload in your new class is type remapping. No need to define a ‘run()’ or to overload ‘postProcess()’ or ‘visit()’.

[dan-zheng]

My notes:

• SILCloner::cloneFunction does clone function arguments, but is not suitable for TypeSubstCloner because it does not remap argument types.
  • Perhaps a good patch would be to add something like TypeSubstCloner::cloneFunction upstream that does BasicTypeSubstCloner::run(). AFAIK currently all subclasses duplicate the same "create entry and function arguments" logic. For now, I defined BasicTypeSubstCloner::run() for convenience.
• Overriding TypeSubstCloner::postProcess is necessary:

  void postProcess(SILInstruction *Orig, SILInstruction *Cloned) {
    llvm_unreachable("Clients need to explicitly call a base class impl!");
  }
  

• Not overriding TypeSubstCloner::visit is fine, thanks!

Please let me know if you have suggestions.

[rxwei]

Great. It’d be good to include what the original problem was and this context in the PR message.

[dan-zheng]

Updated PR description, thanks!

[dan-zheng]

@swift-ci Please test tensorflow