[Bug c++/71391] New: error on aggregate initialization with side-effects in a constexpr function

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71391

            Bug ID: 71391
           Summary: error on aggregate initialization with side-effects in
                    a constexpr function
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

All versions of GCC reject the following valid C++ 14 program.  (Clang and EDG
accept it as expected.)

$ cat zzz.cpp && /home/msebor/build/gcc-6-branch/gcc/xgcc -B
/home/msebor/build/gcc-6-branch/gcc -S -Wall -Wextra -Wpedantic zzz.cpp 
struct S { int a, b; };

constexpr S f ()
{
  int x = 0;
  S z = { 0, ++x }; 
  return z;
}

constexpr S s = f ();

static_assert (s.a + 1 == s.b, "");
zzz.cpp:12:1: error: non-constant condition for static assertion
 static_assert (s.a + 1 == s.b, "");
 ^~~~~~~~~~~~~
zzz.cpp:12:1: error: the value of ‘s’ is not usable in a constant expression
zzz.cpp:10:13: note: ‘s’ used in its own initializer
 constexpr S s = f ();
             ^