[Bug c++/81853] New: [C++14] "using namespace" is not a constant expression in function-like macro

["t.poechtrager at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81853

            Bug ID: 81853
           Summary: [C++14] "using namespace" is not a constant expression
                    in function-like macro
           Product: gcc
           Version: 7.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: t.poechtrager at gmail dot com
  Target Milestone: ---

$ g++ --version
g++ (GCC) 7.1.1 20170630
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

---------------------------------------------------------------------------

$ cat test.cpp 

namespace test {
constexpr int test_fun() { return 42; }
}

#define test_macro() \
({ \
    using namespace test; \
    test_fun(); \
})

int main()
{
    constexpr int test_var = test_macro();
    return test_var;
}

---------------------------------------------------------------------------

$ g++ test.cpp 
test.cpp: In function ‘int main()’:
test.cpp:8:19: error: statement is not a constant expression
   using namespace test; \
                   ^
test.cpp:14:30: note: in expansion of macro ‘test_macro’
     constexpr int test_var = test_macro();
                              ^~~~~~~~~~

---------------------------------------------------------------------------

Clang accepts this since 3.4:

$ clang++ test.cpp -std=c++14 && ./a.out ; echo $?
42