[all-commits] [llvm/llvm-project] 9cfb13: [Clang][Sema] Defer instantiation of exception spe...

[Krystian Stasiowski via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 9cfb138eccb83b5876928b08be346fde5ca78b47
      https://github.com/llvm/llvm-project/commit/9cfb138eccb83b5876928b08be346fde5ca78b47
  Author: Krystian Stasiowski <sdkrystian at gmail.com>
  Date:   2024-02-26 (Mon, 26 Feb 2024)

  Changed paths:
    M clang/docs/ReleaseNotes.rst
    M clang/lib/Sema/SemaTemplate.cpp
    M clang/lib/Sema/SemaTemplateDeduction.cpp
    A clang/test/CXX/except/except.spec/p13.cpp
    M clang/test/SemaTemplate/class-template-noexcept.cpp

  Log Message:
  -----------
  [Clang][Sema] Defer instantiation of exception specification until after partial ordering when determining primary template (#82417)

Consider the following:
```
struct A {
  static constexpr bool x = true;
};

template<typename T, typename U>
void f(T, U) noexcept(T::y); // #1, error: no member named 'y' in 'A'

template<typename T, typename U>
void f(T, U*) noexcept(T::x); // #2

template<>
void f(A, int*) noexcept; // explicit specialization of #2
```

We currently instantiate the exception specification of all candidate
function template specializations when deducting template arguments for
an explicit specialization, which results in a error despite `#1` not
being selected by partial ordering as the most specialized template.
According to [except.spec] p13:
> An exception specification is considered to be needed when: 
> - [...]
> - the exception specification is compared to that of another
declaration (e.g., an explicit specialization or an overriding virtual
function);

Assuming that "comparing declarations" means "determining whether the
declarations correspond and declare the same entity" (per [basic.scope.scope] p4 and
[basic.link] p11.1, respectively), the exception specification does _not_ need to be
instantiated until _after_ partial ordering, at which point we determine
whether the implicitly instantiated specialization and the explicit
specialization declare the same entity (the determination of whether two
functions/function templates correspond does not consider the exception
specifications).

This patch defers the instantiation of the exception specification until
a single function template specialization is selected via partial
ordering, matching the behavior of GCC, EDG, and
MSVC: see https://godbolt.org/z/Ebb6GTcWE.



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