[all-commits] [llvm/llvm-project] 34ae22: [Clang][Sema] Improve support for explicit special...

[Krystian Stasiowski via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 34ae2265e88c8a04350de5a244d0d888e74a8388
      https://github.com/llvm/llvm-project/commit/34ae2265e88c8a04350de5a244d0d888e74a8388
  Author: Krystian Stasiowski <sdkrystian at gmail.com>
  Date:   2024-05-07 (Tue, 07 May 2024)

  Changed paths:
    M clang/docs/ReleaseNotes.rst
    M clang/include/clang/Basic/DiagnosticSemaKinds.td
    M clang/include/clang/Sema/Sema.h
    M clang/lib/Sema/SemaConcept.cpp
    M clang/lib/Sema/SemaOverload.cpp
    M clang/lib/Sema/SemaTemplate.cpp
    M clang/lib/Sema/SemaTemplateDeduction.cpp
    M clang/lib/Sema/SemaTemplateInstantiate.cpp
    A clang/test/CXX/temp/temp.spec/temp.expl.spec/p14-23.cpp
    A clang/test/CXX/temp/temp.spec/temp.expl.spec/p8.cpp

  Log Message:
  -----------
  [Clang][Sema] Improve support for explicit specializations of constrained member functions & member function templates (#88963)

Consider the following snippet from the discussion of CWG2847 on the core reflector:
```
template<typename T>
concept C = sizeof(T) <= sizeof(long);

template<typename T>
struct A 
{
    template<typename U>
    void f(U) requires C<U>; // #1, declares a function template 

    void g() requires C<T>; // #2, declares a function

    template<>
    void f(char);  // #3, an explicit specialization of a function template that declares a function
};

template<>
template<typename U>
void A<short>::f(U) requires C<U>; // #4, an explicit specialization of a function template that declares a function template

template<>
template<>
void A<int>::f(int); // #5, an explicit specialization of a function template that declares a function

template<>
void A<long>::g(); // #6, an explicit specialization of a function that declares a function
```

A number of problems exist:
- Clang rejects `#4` because the trailing _requires-clause_ has `U`
substituted with the wrong template parameter depth when
`Sema::AreConstraintExpressionsEqual` is called to determine whether it
matches the trailing _requires-clause_ of the implicitly instantiated
function template.
- Clang rejects `#5` because the function template specialization
instantiated from `A<int>::f` has a trailing _requires-clause_, but `#5`
does not (nor can it have one as it isn't a templated function).
- Clang rejects `#6` for the same reasons it rejects `#5`.

This patch resolves these issues by making the following changes:
- To fix `#4`, `Sema::AreConstraintExpressionsEqual` is passed
`FunctionTemplateDecl`s when comparing the trailing _requires-clauses_
of `#4` and the function template instantiated from `#1`.
- To fix `#5` and `#6`, the trailing _requires-clauses_ are not compared
for explicit specializations that declare functions.

In addition to these changes, `CheckMemberSpecialization` now considers
constraint satisfaction/constraint partial ordering when determining
which member function is specialized by an explicit specialization of a
member function for an implicit instantiation of a class template (we
previously would select the first function that has the same type as the
explicit specialization). With constraints taken under consideration, we
match EDG's behavior for these declarations.



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