This patch makes the bind placeholders in std::placeholders both (1) const and
(2) constexpr (See below).
This is technically a breaking change for any code using the placeholders
outside of std::bind and depending on them being non-const. However I don't
think this will break any real world code.
(1) Previously the placeholders were non-const extern globals in all
dialects. This patch changes these extern globals to be const in all dialects.
Since the cv-qualifiers don't participate in name mangling for globals this
is an ABI compatible change.
(2) Make the placeholders constexpr in C++11 and beyond. Although LWG 2488 only
applies to C++17 I don't see any reason not to backport this change.
llvm-svn: 273824
