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From: "davidfromonline at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/106211] New: Rejects valid with function template with
 non-deduced parameters from deduced parameters of another function template
Date: Wed, 06 Jul 2022 03:31:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106211

            Bug ID: 106211
           Summary: Rejects valid with function template with non-deduced
                    parameters from deduced parameters of another function
                    template
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: davidfromonline at gmail dot com
  Target Milestone: ---

The following valid translation unit

```
template<typename, typename>
concept any =3D true;

void f() {
        []<typename... Ts>(Ts...) {
                return [](any<Ts> auto..., auto) {};
        }(0)(0, 0);
}
```

is rejected by gcc with the error message:

```
<source>: In function 'void f()':
<source>:7:13: error: no match for call to '(f()::<lambda(Ts
...)>::<lambda(auto:1 ..., auto:2)>) (int, int)'
    5 |         []<typename... Ts>(Ts...) {
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    6 |                 return [](any<Ts> auto..., auto) {};
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    7 |         }(0)(0, 0);
      |         ~~~~^~~~~~
<source>:6:24: note: candidate: 'template<class ... auto:1, class auto:2>=20
requires (... && any<auto:1, Ts>) f()::<lambda(Ts ...)>::<lambda(auto:1 ...,
auto:2)>'
    6 |                 return [](any<Ts> auto..., auto) {};
      |                        ^
<source>:6:24: note:   template argument deduction/substitution failed:
<source>:7:13: note:   candidate expects 1 argument, 2 provided
    5 |         []<typename... Ts>(Ts...) {
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    6 |                 return [](any<Ts> auto..., auto) {};
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    7 |         }(0)(0, 0);
      |         ~~~~^~~~~~
Compiler returned: 1
```

See it live: https://godbolt.org/z/c7zxWEbxM=