[Bug c++/80485] New: rejects-valid: constexpr static_cast of pointer-to-member-function to bool

["TonyELewis at hotmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80485

            Bug ID: 80485
           Summary: rejects-valid: constexpr static_cast of
                    pointer-to-member-function to bool
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: TonyELewis at hotmail dot com
  Target Milestone: ---

GCC rejects the following code (partly adapted from Boost's tribool), which I
think is valid:


struct dummy {
  void nonnull() {};
};

typedef void (dummy::*safe_bool)();

constexpr safe_bool a = &dummy::nonnull;

static_assert( static_cast<bool>( a ), "" );

int main () { return 0; }


I'm compiling with -std=c++14 and I'm seeing these errors:


<source>:9:1: error: non-constant condition for static assertion
 static_assert( static_cast<bool>( a ), "" );
 ^~~~~~~~~~~~~
<source>:9:16: error: '(dummy::nonnull != 0)' is not a constant expression
 static_assert( static_cast<bool>( a ), "" );
                ^~~~~~~~~~~~~~~~~~~~~~
Compiler exited with result code 1


This code is accepted by Clang.

This looks related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71896 but
that has status "RESOLVED FIXED" but I'm seeing this on "GCC 7 (snapshot)" on
godbolt.org (see https://godbolt.org/g/JnrZss ) as well as on my GCC 6.2.0.