[Bug c++/78701] New: Template deduction, dependent template and conversion to bool failure

[vince.rev at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78701

            Bug ID: 78701
           Summary: Template deduction, dependent template and conversion
                    to bool failure
           Product: gcc
           Version: 6.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vince.rev at gmail dot com
  Target Milestone: ---

The following code fails to compile on g++ 6.1, 6.2, and 7 (and I guess on g++
6.0):
======================================================================
#include <iostream>
template <class T, T N = T(), bool B = N + 1> T f(T x) {return T();}
int main(int argc, char* argv[]) {return f(42);}
======================================================================

It returns the following error:
======================================================================
test_bug.cpp: In function ‘int main(int, char**)’:
test_bug.cpp:3:46: error: no matching function for call to ‘f(int)’
 int main(int argc, char* argv[]) {return f(42);}
                                              ^
test_bug.cpp:2:49: note: candidate: template<class T, T N, bool B> T f(T)
 template <class T, T N = T(), bool B = N + 1> T f(T x) {return T();}
                                                 ^
test_bug.cpp:2:49: note:   template argument deduction/substitution failed:
test_bug.cpp:2:42: error: invalid use of template type parameter ‘T’
 template <class T, T N = T(), bool B = N + 1> T f(T x) {return T();}
                                        ~~^~~
test_bug.cpp:2:44: error: could not convert template argument ‘<expression
error>’ to ‘bool’
 template <class T, T N = T(), bool B = N + 1> T f(T x) {return T();}
                                            ^
======================================================================