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As proposed already for Qt 5 (then with q14 and q17), add
reimplementations of C++20 library features into a q20 namespace. The
advantage is that we can later just s/q20::/std::/ and be sure that a)
the q20 functionality works (within reason) like the std counterparts
and b) we don't have to re-indent the code after the replacement.
Start with std::identity in q20functional.h, required by
std::ranges::{any,all,none}_of() (ex ranges[1]) in q20algorithm.h,
which I happen to require in QLibrary next.
[1] We can't provide the nice range-based overloads (any_of(vector,
pred)), yet, because that would require to reimplement all the
range-related concepts, as the overloads are ambiguous if
unconstrained. First, we should check whether we can't just depend on
Nieber's ranges-v3 library instead. The q20 namespace is independent
of this decision, as it's more universally useful (q20::remove_cvref
comes to mind).
Unlike in q14/q17 times, a single header, q20.h, would become unwieldy
very soon, so use separate headers.
Change-Id: I14b98c865e242d2dc9674bc1bff5f7a9e4db9940
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
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