commit 8c852ab57932a5cd954cb0d050c3d2ab486428df
author Younan Zhang <zyn7109@gmail.com> Fri May 10 20:47:15 2024 +0800
committer GitHub <noreply@github.com> Fri May 10 20:47:15 2024 +0800
tree 2c05c9bf4f07312f353497efd5144c9738fbad2d
parent 200f3bd39562f4d605f13567398025d30fa27d61

[Clang][Sema] Revise the transformation of CTAD parameters of nested class templates (#91628)

This fixes a regression introduced by bee78b88f.

When we form a deduction guide for a constructor, basically, we do the
following work:
- Collect template parameters from the constructor's surrounding class
template, if present.
- Collect template parameters from the constructor.
- Splice these template parameters together into a new template
parameter list.
- Turn all the references (e.g. the function parameter list) to the
invented parameter list by applying a `TreeTransform` to the function
type.

In the previous fix, we handled cases of nested class templates by
substituting the "outer" template parameters (i.e. those not declared at
the surrounding class template or the constructor) with the
instantiating template arguments. The approach per se makes sense, but
there was a flaw in the following case:

```cpp
template <typename U, typename... Us> struct X {
  template <typename V> struct Y {
    template <typename T> Y(T) {}
  };

  template <typename T> Y(T) -> Y<T>;
};

X<int>::Y y(42);
```

While we're transforming the parameters for `Y(T)`, we first attempt to
transform all references to `V` and `T`; then, we handle the references
to outer parameters `U` and `Us` using the template arguments from
`X<int>` by transforming the same `ParamDecl`. However, the first step
results in the reference `T` being `<template-param-0-1>` because the
invented `T` is the last of the parameter list of the deduction guide,
and what we're substituting with is a corresponding parameter pack
(which is `Us`, though empty). Hence we're messing up the substitution.

I think we can resolve it by reversing the substitution order, which
means handling outer template parameters first and then the inner
parameters.

There's no release note because this is a regression in 18, and I hope
we can catch up with the last release.

Fixes https://github.com/llvm/llvm-project/issues/88142