[Bug c++/105268] New: type/value mismatch when using variadic concept

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105268

            Bug ID: 105268
           Summary: type/value mismatch when using variadic concept
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

>From StackOverflow (https://stackoverflow.com/q/71864544/2069064):

template<typename> concept C_one = true;
template<typename...> concept C_many = true;

template<bool B> struct S { };

template<typename T = S<C_one<int>>> void f();  // ok
template<typename T = S<C_many<int>>> void g(); // error

gcc rejects the declaration of g with:


<source>:7:35: error: type/value mismatch at argument 1 in template parameter
list for 'template<bool B> struct S'
    7 | template<typename T = S<C_many<int>>> void g();
      |                                   ^~
<source>:7:35: note:   expected a constant of type 'bool', got
'<template-parameter-1-1>'

But C_many<int> is a bool, so this should be fine. And template-parameter-1-1
isn't a very useful diagnostic anyway.