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From: "ted at lyncon dot se" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110604] New: template argument deduction failed with
 decltype(lambda)
Date: Sun, 09 Jul 2023 13:47:45 +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: ted at lyncon dot se
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110604

            Bug ID: 110604
           Summary: template argument deduction failed with
                    decltype(lambda)
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ted at lyncon dot se
  Target Milestone: ---

Compiling this in C++20 mode on trunk (https://godbolt.org/z/dW5cf3WrK):
```
template<typename>
struct foo {
    template<typename =3D decltype([]{})>
    void bar() {}
};

int main() {
    [[maybe_unused]] auto v =3D foo<void>();

    v.bar<>();
    v.bar();
}
```
Results in the following error:
```
<source>: In function 'int main()':
<source>:10:12: error: no matching function for call to 'foo<void>::bar<>()'
   10 |     v.bar<>();
      |     ~~~~~~~^~
<source>:4:10: note: candidate: 'template<class> void foo<
<template-parameter-1-1> >::bar() [with <template-parameter-1-1> =3D void]'
    4 |     void bar() {}
      |          ^~~
<source>:4:10: note:   template argument deduction/substitution failed:
<source>:11:10: error: no matching function for call to 'foo<void>::bar()'
   11 |     v.bar();
      |     ~~~~~^~
<source>:4:10: note: candidate: 'template<class> void foo<
<template-parameter-1-1> >::bar() [with <template-parameter-1-1> =3D void]'
    4 |     void bar() {}
      |          ^~~
<source>:4:10: note:   template argument deduction/substitution failed:
```
Possibly a duplicate of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D1027=
21,
but I'm not 100% sure.=