[dcl.constexpr]
Change: constexpr non-static member functions are not implicitly
const member functions.
Rationale: Necessary to allow constexpr member functions to mutate
the object.
Effect on original feature:
Valid C++ 2011 code may fail to compile in this International Standard.
For example, the following code is valid in C++ 2011
but invalid in this International Standard because it declares the same member
function twice with different return types:
struct S {
constexpr const int &f();
int &f();
};