Implement C++20 constexpr <algorithm>, , and <array>

[Jonathan Wakely]

On 17/03/19 22:54 -0400, Ed Smith-Rowland via libstdc++ wrote:
>Greetings,
>
>This patch implements C++20 p0202 - Add Constexpr Modifiers to 
>Functions in <algorithm> and <utility> Headers
>
>and C++20 p1023 - constexpr comparison operators for std::array.
>
>
>The patch is large because of the testsuite additions. Basically, the 
>algorithms and the array comparison operators are all marked constexpr 
>for C++20.  This has been built and tested on x86_64-linux.

As discussed on IRC< the tests need to be run with -std=gnu++2a to
ensure everything we test (which is not exhaustive, but is the best we
can do) still works in C++2a mode.

The indentation went bad here:

   template<typename _FIter1, typename _FIter2, typename _BinaryPredicate>
-    _FIter1
+    _GLIBCXX20_CONSTEXPR
+   _FIter1
     find_first_of(_FIter1, _FIter1, _FIter2, _FIter2, _BinaryPredicate);


I would have to look through old emails but I think there's a reason
the function objects like _Iter_comp_to_val take their arguments by
non-const reference.

I'm very surprised that none of the algos that dispatch to
__builtin_memove need changes, because those optimizations won't work
in constant expressions. I would expect to have to use
std::is_constant_evaluated to disable the optimizations when used in
constant expressions.