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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/81202] Concept parsing error for default template arguments
Date: Sat, 11 Dec 2021 05:00:34 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D81202

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Full testcase:
#include <type_traits>
#include <tuple>

template<class T,class AT=3Dvoid> constexpr bool
IsPtrC=3Dstd::is_pointer_v<T>&&(std::is_same_v<AT,void>
||std::is_same_v<AT,std::remove_pointer<T>>);
template<class T,class AT=3Dvoid> concept  IsPtrC2=3DIsPtrC<T,AT>;
template<bool B> using TEST=3Dint;
int main(int argc,char*argv[]){
  TEST<IsPtrC<int>> i;//OK
  TEST<IsPtrC2<int>> j;//error: parse error in template argument list
  TEST<IsPtrC2<int,void>> k;//OK
  TEST<(IsPtrC2<int>)> l;//OK
}


With -std=3Dc++20 the above code works (as expected) but with -std=3Dc++17
-fconcepts it fails (with GCC 10+).

I will let a C++ maintainer to decide if it should work with -std=3Dc++17
-fconcepts or not.

The error message is:
<source>: In function 'int main(int, char**)':
<source>:10:19: error: type/value mismatch at argument 1 in template parame=
ter
list for 'template<bool B> using TEST =3D int'
   10 |   TEST<IsPtrC2<int>> j;//error: parse error in template argument li=
st
      |                   ^~
<source>:10:19: note:   expected a constant of type 'bool', got 'auto [requ=
ires
IsPtrC2<<placeholder>, int>]'
Compiler returned: 1=