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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/107649] New: New std::complex specializations are
 never used
Date: Fri, 11 Nov 2022 20:46:22 +0000
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X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 13.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107649

            Bug ID: 107649
           Summary: New std::complex specializations are never used
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

In <complex> we have:

   template<typename _Tp>
     requires(__complex_type<_Tp>::type)
     class complex<_Tp>
     {

This partial specialization is never used. The constraint looks for a const=
ant
expression with true value, and none of the __complex_type specializations
provide that.

To test for a type name the constraint must be:


  template<typename _Tp>
    requires requires { typename __complex_type<_Tp>::type; }
    class complex<_Tp>
    {


However if I fix this, so that the partial specialization is used for float=
64_t
etc., then I get a cascade of errors for every overloaded <math.h> function:


include/complex: In instantiation of '_Tp std::abs(const complex<_Tp>&) [wi=
th
_Tp =3D _Float16]':
testsuite/26_numerics/complex/ext_c++23.cc:28:   required from 'void
test_functions(T*, std::complex<_Tp>*) [with T =3D _Float16]'
testsuite/26_numerics/complex/ext_c++23.cc:56:   required from here
include/complex:629: error: call of overloaded
'__complex_abs(std::complex<_Float16>::_ComplexT)' is ambiguous
include/complex:618: note: candidate: 'float std::__complex_abs(__complex__
float)'
include/complex:621: note: candidate: 'double std::__complex_abs(__complex__
double)'
include/complex:624: note: candidate: 'long double std::__complex_abs(const
__complex__ long double&)'
include/complex: In instantiation of '_Tp std::arg(const complex<_Tp>&) [wi=
th
_Tp =3D _Float16]':
testsuite/26_numerics/complex/ext_c++23.cc:29:   required from 'void
test_functions(T*, std::complex<_Tp>*) [with T =3D _Float16]'
testsuite/26_numerics/complex/ext_c++23.cc:56:   required from here
include/complex:656: error: call of overloaded
'__complex_arg(std::complex<_Float16>::_ComplexT)' is ambiguous
include/complex:645: note: candidate: 'float std::__complex_arg(__complex__
float)'
include/complex:648: note: candidate: 'double std::__complex_arg(__complex__
double)'
include/complex:651: note: candidate: 'long double std::__complex_arg(const
__complex__ long double&)'
include/complex: In instantiation of 'std::complex<_Tp> std::cos(const
complex<_Tp>&) [with _Tp =3D _Float16]':
testsuite/26_numerics/complex/ext_c++23.cc:30:   required from 'void
test_functions(T*, std::complex<_Tp>*) [with T =3D _Float16]'
testsuite/26_numerics/complex/ext_c++23.cc:56:   required from here


This is because with the fix to the speclization, __rep() now returns a
__complex__ T and so we try to call one of the float/double/longdouble
overloads:

  inline float
  __complex_abs(__complex__ float __z) { return __builtin_cabsf(__z); }

  inline double
  __complex_abs(__complex__ double __z) { return __builtin_cabs(__z); }

  inline long double
  __complex_abs(const __complex__ long double& __z)
  { return __builtin_cabsl(__z); }

  template<typename _Tp>
    inline _Tp
    abs(const complex<_Tp>& __z) { return __complex_abs(__z.__rep()); }=