[Bug c++/84489] New: Non-type template parameter dependency

[vince.rev at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84489

            Bug ID: 84489
           Summary: Non-type template parameter dependency
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vince.rev at gmail dot com
  Target Milestone: ---

Created attachment 43475
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43475&action=edit
Example code of the bug

Consider the following code in C++11.
=========================================================
#include <iostream>

template <class T = int, T N = T(), T B = (N >> 1)>
constexpr T f0() {return 0;}

template <class T = int, T N = T(), bool B = (N >> 1)>
constexpr T f1() {return 0;}

int main()
{
    f0(); // No bug
    f1(); // Bug here
    return 0;
}
=========================================================

It compiles well under g++ 4.7, 4.8, 4.9, 5.X but fails to compile under 6.X
and 7.X. It also compiles well under clang. The returned error is:

=========================================================
gcc_bug_template_bool.cpp: In function 'int main()':
gcc_bug_template_bool.cpp:12:8: error: no matching function for call to 'f1()'
     f1();
        ^
gcc_bug_template_bool.cpp:7:13: note: candidate: template<class T, T N, bool B>
constexpr T f1()
 constexpr T f1() {return 0;}
             ^~
gcc_bug_template_bool.cpp:7:13: note:   template argument
deduction/substitution failed:
gcc_bug_template_bool.cpp:6:49: error: invalid use of template type parameter
'T'
 template <class T = int, T N = T(), bool B = (N >> 1)>
                                              ~~~^~~~~
gcc_bug_template_bool.cpp:6:53: error: could not convert template argument
'<expression error>' from '<type error>' to 'bool'
 template <class T = int, T N = T(), bool B = (N >> 1)>
=========================================================