gh-356-cpp14-constexpr-func-in-cpp11.chng.txt

[GH #356]
GNU/Clang compatibility: C++11 constexpr templates with non-return statements

When a statement that is not permitted in a C++11 constexpr function or
constructor was accepted with a warning (for example, in a function or
constructor template in some GNU or Clang C++11 modes), the function or
constructor was nevertheless marked as unusable in constant expressions.
That is now fixed: The function or constructor remains eligible for constant
evaluation unless the diagnostic is issued as an error.  For example, with
--c++11 --gnu=150000:

  template<typename T> struct S {
    T x;
    constexpr S(T i) : x(i) {
      if (sizeof(T) == 0) {}  // Warning in C++11 mode.
    }
  };
  constexpr S<int> s(42);  // Previously an error, now okay.