[Bug c++/81933] New: [7 Regression] Invalid "constexpr call flows off the end of the function" error

["ldionne.2 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81933

            Bug ID: 81933
           Summary: [7 Regression] Invalid "constexpr call flows off the
                    end of the function" error
           Product: gcc
           Version: 7.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldionne.2 at gmail dot com
  Target Milestone: ---

The following code does not compile with GCC 7, std=c++14:

----------------------------------------------------------------------------
    #include <tuple>

    struct any_udt { };

    template <typename ...Tuples>
    constexpr auto flatten(Tuples ...tuples) {
      auto all = std::make_tuple(tuples...);
      auto flat = std::make_tuple(
        std::get<0>(std::get<1>(all)),
        std::get<0>(std::get<2>(all))
      );
      return flat;
    }

    constexpr auto fail = ::flatten(std::make_tuple(),
                                    std::make_tuple(any_udt{}),
                                    std::make_tuple(any_udt{}));

    int main() { }
----------------------------------------------------------------------------

It triggers the following error:

    foo.cpp:17:59:   in constexpr expansion of 'flatten(Tuples ...) 
      [with Tuples = {std::tuple<>, std::tuple<any_udt>, std::tuple<any_udt>}]
    (std::make_tuple(_Elements&& ...) [with _Elements = {any_udt}](), 
     std::make_tuple(_Elements&& ...) [with _Elements = {any_udt}]())'

    foo.cpp:17:59: error: constexpr call flows off the end of the function
                                     std::make_tuple(any_udt{}));
                                                               ^


Note that the code compiles just fine with GCC 6 and all Clang versions I've
tried, so this looks like a 7 regression.

Working example (GCC 6.3.0, -std=c++14):
https://wandbox.org/permlink/5AjQ50OSxNjJPP8r
Working example (GCC 6.3.0, -std=c++1z):
https://wandbox.org/permlink/dKegxu0QeVXtzObB

Failing example (GCC 7.2.0, -std=c++14):
https://wandbox.org/permlink/hfbI0Ndg5UyCIF9W
Working example (GCC 7.2.0, -std=c++1z):
https://wandbox.org/permlink/Hz1gm6EipU5Ej5jy

Failing example (GCC 8.0.0, -std=c++14):
https://wandbox.org/permlink/jSHOPt8zXosccp8a
Working example (GCC 8.0.0, -std=c++1z):
https://wandbox.org/permlink/Nj7tNIYWu8Ueury7