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

[msebor at gcc dot gnu.org]

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

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
Including the following test case from the duplicate bug 71116 (which is
expected to pass in C+++ 14 and prior):

$ cat uu.cpp && /home/msebor/build/gcc-fortify-source/gcc/xgcc
-B/home/msebor/build/gcc-fortify-source/gcc -S uu.cpp
void oops () {
  auto fn = []{ };
  static_assert (!__is_literal_type (decltype (fn)), "oops");
}


uu.cpp: In function ‘void oops()’:
uu.cpp:3:3: error: static assertion failed: oops
   static_assert (!__is_literal_type (decltype (fn)), "oops");
   ^~~~~~~~~~~~~