[Bug libstdc++/106067] New: Error on implicit conversion to std::function: 'static constexpr bool std::_Function_handler<...>...' used before its definition

[e.tadeu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106067

            Bug ID: 106067
           Summary: Error on implicit conversion to std::function: 'static
                    constexpr bool std::_Function_handler<...>...' used
                    before its definition
           Product: gcc
           Version: 11.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: e.tadeu at gmail dot com
  Target Milestone: ---

The following code started failing on GCC 11.3 (also fails on GCC 12):

    #include <functional>

    double fn();

    template <double (*T)(std::function<double()>)>
    void caller()
    {
        auto f = T(fn);
        // auto f = T(std::function<double()>(fn));  // workaround (this works)
    }


The snippet is also here: https://godbolt.org/z/zh7nr7vKc

The full error is:

        In file included from
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/move.h:57,
                                         from
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/stl_function.h:60,
                                         from
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/functional:49,
                                         from <source>:1:
        /opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/type_traits: In
instantiation of 'struct std::is_trivially_copyable<double (*)()>':
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:69:12:
  required from 'struct std::__is_location_invariant<double (*)()>'
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:125:38:
  required from 'const bool std::_Function_base::_Base_manager<double
(*)()>::__stored_locally'
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:130:8:
  required from 'class std::_Function_base::_Base_manager<double (*)()>'
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:260:11:
  required from 'class std::_Function_handler<double(), double (*)()>'
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:436:65:
  required from 'std::function<_Res(_ArgTypes ...)>::function(_Functor&&) [with
_Functor = double (*)(); _Constraints = void; _Res = double; _ArgTypes = {}]'
        <source>:8:15:   required from here
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/type_traits:713:52: error:
non-constant condition for static assertion
          713 |      
static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
                  |                    
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/type_traits:713:52: error:
'constexpr std::true_type
std::__is_complete_or_unbounded(std::__type_identity<_Tp>) [with _Tp = double
(*)(); long unsigned int <anonymous> = 8; std::true_type =
std::integral_constant<bool, true>]' used before its definition
        In file included from
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/functional:59,
                                         from <source>:1:
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h: In
instantiation of 'std::function<_Res(_ArgTypes ...)>::function(_Functor&&)
[with _Functor = double (*)(); _Constraints = void; _Res = double; _ArgTypes =
{}]':
        <source>:8:15:   required from here
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:436:72:
error: 'static constexpr bool std::_Function_handler<_Res(_ArgTypes ...),
_Functor>::_S_nothrow_init() [with _Fn = double (*)(); _Res = double; _Functor
= double (*)(); _ArgTypes = {}]' used before its definition
          436 |         noexcept(_Handler<_Functor>::template
_S_nothrow_init<_Functor>())
                  |                 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
        ASM generation compiler returned: 1
        In file included from
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/move.h:57,
                                         from
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/stl_function.h:60,
                                         from
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/functional:49,
                                         from <source>:1:
        /opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/type_traits: In
instantiation of 'struct std::is_trivially_copyable<double (*)()>':
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:69:12:
  required from 'struct std::__is_location_invariant<double (*)()>'
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:125:38:
  required from 'const bool std::_Function_base::_Base_manager<double
(*)()>::__stored_locally'
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:130:8:
  required from 'class std::_Function_base::_Base_manager<double (*)()>'
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:260:11:
  required from 'class std::_Function_handler<double(), double (*)()>'
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:436:65:
  required from 'std::function<_Res(_ArgTypes ...)>::function(_Functor&&) [with
_Functor = double (*)(); _Constraints = void; _Res = double; _ArgTypes = {}]'
        <source>:8:15:   required from here
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/type_traits:713:52: error:
non-constant condition for static assertion
          713 |      
static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
                  |                    
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/type_traits:713:52: error:
'constexpr std::true_type
std::__is_complete_or_unbounded(std::__type_identity<_Tp>) [with _Tp = double
(*)(); long unsigned int <anonymous> = 8; std::true_type =
std::integral_constant<bool, true>]' used before its definition
        In file included from
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/functional:59,
                                         from <source>:1:
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h: In
instantiation of 'std::function<_Res(_ArgTypes ...)>::function(_Functor&&)
[with _Functor = double (*)(); _Constraints = void; _Res = double; _ArgTypes =
{}]':
        <source>:8:15:   required from here
       
/opt/compiler-explorer/gcc-11.3.0/include/c++/11.3.0/bits/std_function.h:436:72:
error: 'static constexpr bool std::_Function_handler<_Res(_ArgTypes ...),
_Functor>::_S_nothrow_init() [with _Fn = double (*)(); _Res = double; _Functor
= double (*)(); _ArgTypes = {}]' used before its definition
          436 |         noexcept(_Handler<_Functor>::template
_S_nothrow_init<_Functor>())
                  |                 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
        Execution build compiler returned: 1