[all-commits] [llvm/llvm-project] ff3f41: [clang] Implement CWG2398 provisional TTP matching...

[Matheus Izvekov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ff3f41deb04c03ba57658776e4e0dc26ef01187d
      https://github.com/llvm/llvm-project/commit/ff3f41deb04c03ba57658776e4e0dc26ef01187d
  Author: Matheus Izvekov <mizvekov at gmail.com>
  Date:   2024-05-22 (Wed, 22 May 2024)

  Changed paths:
    M clang/lib/Sema/SemaTemplate.cpp
    M clang/lib/Sema/SemaTemplateDeduction.cpp
    M clang/test/CXX/temp/temp.decls/temp.alias/p2.cpp
    M clang/test/SemaTemplate/cwg2398.cpp

  Log Message:
  -----------
  [clang] Implement CWG2398 provisional TTP matching to class templates (#92855)

This solves some ambuguity introduced in P0522 regarding how template
template parameters are partially ordered, and should reduce the
negative impact of enabling `-frelaxed-template-template-args` by
default.

When performing template argument deduction, we extend the provisional
wording introduced in https://github.com/llvm/llvm-project/pull/89807 so
it also covers deduction of class templates.

Given the following example:
```C++
template <class T1, class T2 = float> struct A;
template <class T3> struct B;

template <template <class T4> class TT1, class T5> struct B<TT1<T5>>;   // #1
template <class T6, class T7>                      struct B<A<T6, T7>>; // #2

template struct B<A<int>>;
```
Prior to P0522, `#2` was picked. Afterwards, this became ambiguous. This
patch restores the pre-P0522 behavior, `#2` is picked again.

This has the beneficial side effect of making the following code valid:
```C++
template<class T, class U> struct A {};
A<int, float> v;
template<template<class> class TT> void f(TT<int>);

// OK: TT picks 'float' as the default argument for the second parameter.
void g() { f(v); }
```

---

Since this changes provisional implementation of CWG2398 which has not
been released yet, and already contains a changelog entry, we don't
provide a changelog entry here.



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