[Bug c++/70979] [C++14] g++ falsely accepts constexpr function returning a lambda

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70979

--- Comment #6 from Jason Merrill <jason at gcc dot gnu.org> ---
Created attachment 40731
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40731&action=edit
patch for DR 1071

Relatedly, here's a patch to implement DR 1071, which is the error clang gives.
 But this breaks several things in libstdc++, so it needs more investigation
before it will be ready even for GCC 8.