The NFC part of D116809. We still want to enforce this in CI,
but the mechanism for that is still to-be-determined.
Differential Revision: https://reviews.llvm.org/D116809
Before this patch, the user needed to specialize both of
`is_placeholder<MyType>` and `is_placeholder<const MyType>`.
After this patch, only the former is needed (although the
latter is harmless if provided).
The new tests don't actually fail unless return type deduction
is used, which is a C++14 feature. Specializing `is_placeholder`
is still allowed in C++11, though.
Fixes#51095.
Differential Revision: https://reviews.llvm.org/D116388
All supported compilers provide support for inline variables in C++17 now.
Also, as a fly-by fix, replace some uses of _LIBCPP_CONSTEXPR by just
constexpr.
The only exception in this patch is `std::ignore`, which is provided
prior to C++17. Since it is defined in an anonymous namespace, it always
has internal linkage anyway, so using an inline variable there doesn't
provide any benefit. Instead, `inline` was removed entirely on `std::ignore`.
Differential Revision: https://reviews.llvm.org/D110243
In other places in the code, we use lowercase spelling for things that
are not available in prior standards.
Differential Revision: https://reviews.llvm.org/D109435
Arthur O'Dwyer