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From: "ed at catmur dot uk" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/112354] New: mismatched types 'B<T>' and 'B<int>&' for
 generic lambda noexcept-specifier referencing enclosing function parameter
Date: Thu, 02 Nov 2023 18:24:36 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 14.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: ed at catmur dot uk
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112354

            Bug ID: 112354
           Summary: mismatched types 'B<T>' and 'B<int>&' for generic
                    lambda noexcept-specifier referencing enclosing
                    function parameter
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

Since 4.9.0,

template<class>
struct B {
    B();
    B(B const&);
};
template<class T> void g(B<T>, int);
auto f(B<int> x) {
    return [](auto y) noexcept(noexcept(g(x, y))) {
        return 1;
    };
}
int i =3D f({})(0);

fails with:

<source>: In instantiation of 'f(B<int>)::<lambda(auto:1)> [with auto:1 =3D
int]':
<source>:12:14:   required from here
   12 | int i =3D f({})(0);
      |         ~~~~~^~~
<source>:8:42: error: no matching function for call to 'g(B<int>&&, int&)'
    8 |     return [](auto y) noexcept(noexcept(g(x, y))) {
      |                                         ~^~~~~~
<source>:6:24: note: candidate: 'template<class T> void g(B<T>, int)'
    6 | template<class T> void g(B<T>, int);
      |                        ^
<source>:6:24: note:   template argument deduction/substitution failed:
<source>:8:42: note:   mismatched types 'B<T>' and 'B<int>&'
    8 |     return [](auto y) noexcept(noexcept(g(x, y))) {
      |                                         ~^~~~~~=