Some static analysis tools incorrectly identify this constructor
as leaving _has_value in an undefined state. Explicitly call the
default constructor to force _has_value to false.
#pragma once is a widely supported compiler pragma, even though it is
not part of the C++ standard. Many of the issues keeping #pragma once
from being standardized (distributed filesystems, build farms, hard
links, etc.) do not apply to CMake - it is easy to build CMake on a
single machine. CMake also does not install any header files which can
be consumed by other projects (though cmCPluginAPI.h has been
deliberately omitted from this conversion in case anyone is still using
it.) Finally, #pragma once has been required to build CMake since at
least August 2017 (7f29bbe6 enabled server mode unconditionally, which
had been using #pragma once since September 2016 (b13d3e0d)). The fact
that we now require C++11 filters out old compilers, and it is unlikely
that there is a compiler which supports C++11 but does not support
#pragma once.
Previously, cm::static_reference_cast used invoke_result_t and took the
address of O::get. This is not in complete conformance with standard.
This MR changes the implementation to use std::declval<O>.get() which is
always well-defined.
For now, compiler extensions are no longer activated on CMake sources.
However these extensions are still used for various third parties.
This MR is a partial answer to the issue #20454.
Provide a standardized way to handle the C++ "standard" headers
customized to be used with current CMake C++ standard constraints.
Offer under directory `cm` headers which can be used as direct
replacements of the standard ones. For example:
#include <cm/string_view>
can be used safely for CMake development in place of the `<string_view>`
standard header.
Fixes: #19491
