[Bug c++/78551] New: Internal compiler error with constexpr initialization of union

[vlad at petric dot cc]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78551

            Bug ID: 78551
           Summary: Internal compiler error with constexpr initialization
                    of union
           Product: gcc
           Version: 6.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vlad at petric dot cc
  Target Milestone: ---

The following code ICEs:

#include <array>
#include <cinttypes>

struct A {
  union {
    uint64_t s;
    char buff[8];
  };

  constexpr A(const char* str) : s(0) {
    if (str == nullptr) return;
    size_t i = 0;
    for (; i < 8 && str[i] != '\0'; ++i) {
      buff[i] = str[i];
    }
    if (str[i] != '\0') throw std::out_of_range("");
  }
};

constexpr std::array<A, 2> array({"long", "longer"});

It doesn't ICE if I do any of the following:

1. Switch the constructor to char[N] (where N is from template<size_t N>)
2. Get rid of constexpr
3. Get rid of the union - just initialize buff