[Bug c++/98077] New: C++ 17: Using alias template bug in gcc

[Bug c++/98077] New: C++ 17: Using alias template bug in gcc

[juergen.reiss at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98077

            Bug ID: 98077
           Summary: C++ 17: Using alias template bug in gcc
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: juergen.reiss at gmx dot de
  Target Milestone: ---

The error was reported here:
https://stackoverflow.com/questions/65079471/c-17-using-alias-template-bug-in-gcc
A demo is here: https://godbolt.org/z/avGv3n

[Bug c++/98077] C++ 17: Using alias template bug in gcc

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98077

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2020-12-01
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Please see https://gcc.gnu.org/bugs for the requested info.

[Bug c++/98077] C++ 17: Using alias template bug in gcc

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98077

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW
           Keywords|                            |rejects-valid

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The testcase from the godbolt link is:

// { dg-options "-std=gnu++17" }
#include <functional>

template <typename T>
struct CallableTrait;

template <typename R, typename... Args>
struct CallableTrait<std::function<R(Args...)>>
{
    using ReturnType = R;
};

template <typename Callable>
using CallableTraitT =
CallableTrait<decltype(std::function{std::declval<Callable>()})>;

template <typename Callable>
auto test(Callable&&)
{
    using CallableInfo = CallableTraitT<Callable>;
    static_assert(!std::is_void_v<typename CallableInfo::ReturnType>);
}

int main()
{
    test([]() { return 42; });
}


It requires at least C++17.

[Bug c++/98077] C++ 17: Using alias template bug in gcc

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98077

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced:


template<typename T> T&& declval();

template<typename R> struct function
{
  template<typename T> function(T) { }
};

template<typename T> function(T) -> function<decltype(declval<T>()())>;

template<typename T> constexpr bool is_void_v = false;
template<> constexpr bool is_void_v<void> = true;

template <typename T>
struct CallableTrait;

template <typename R>
struct CallableTrait<function<R>>
{
    using ReturnType = R;
};

template <typename Callable>
using CallableTraitT = CallableTrait<decltype(function{declval<Callable>()})>;

template <typename Callable>
auto test(Callable&&)
{
    using CallableInfo = CallableTraitT<Callable>;
    static_assert(!is_void_v<typename CallableInfo::ReturnType>);
}

int main()
{
    test([]() { return 42; });
}



98077.C: In instantiation of ‘auto test(Callable&&) [with Callable =
main()::<lambda()>]’:
98077.C:34:29:   required from here
98077.C:29:20: error: invalid use of incomplete type ‘struct
CallableTrait<main()::<lambda()> >’
   29 |     static_assert(!is_void_v<typename CallableInfo::ReturnType>);
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
98077.C:14:8: note: declaration of ‘struct CallableTrait<main()::<lambda()> >’
   14 | struct CallableTrait;
      |        ^~~~~~~~~~~~~

[Bug c++/98077] C++ 17: Using alias template bug in gcc

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98077

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org
             Status|NEW                         |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
This is essentially a dup of the older 91911, I think

*** This bug has been marked as a duplicate of bug 91911 ***