libstdc++: Make function_ref non-dangling for stateless wrappers

This patch makes the function_ref non-dangling for the stateless
wrappers:
* any functor for which operator() selected for arguments is static,
* standard functors, including pre-C++20 ones.
In other words, any function_ref fr, that is constructed from stateless
wrapper w, can be still called after the object w is destroyed, e.g.:
  std::function_ref<bool(int, int)> fr(std::ranges::less{});
  fr(1, 2); // OK, previously UB because fr referred to already destroyed
            // temporary
As function_ref's operator() is not constexpr, we test the change by checking
if the above declaration can be made constexpr, as such variable cannot contain
dangling pointer values.

We adjust the function_ref generic constructor from any functor, to use more
specialized invoker:
* _S_static (newly added) if the called operator() overload is static,
  after changes r16-5624-g0ea9d760fbf44c, this covers all post-c++20 functors;
* _S_nttp<_Fd{}> for pre-C++20 standard functors.
In both above cases the value of _M_ptrs is ignored and simply set to nullptr.

This follows same technique (checking _Fd::operator()(args...)), and support
the same set of types, as for one used for the transform views iterators in
r16-5625-g9ed821d107f7a1.

As after this change we provide well-defined behavior for the code, that
previous was undefined, this changes is pure quality-of-implementation.
As illustrated by the test cases, it has observable side effects, where
non-longer dangling constructs can be used to define constexpr function_ref.
However, the standard does not define when the constructors defined constexpr
are actually usable at compile time, and the already have precedent in form
of SSO string for validity such constructs being implementation specific.

libstdc++-v3/ChangeLog:

	* include/bits/funcref_impl.h (function_ref::function_ref(_Fn&&)):
	Use _S_static and _S_nttp invokers.
	* include/bits/funcwrap.h (_Base_invoker::_S_static):
	Define.
	* include/bits/stl_function.h (std::__is_std_op_template)
	(std::__is_std_op_wrapper) [__cplusplus > 201703L]:
	Moved from std/ranges.
	* include/std/ranges (__detail::__is_std_op_template)
	(__detail::__is_std_op_wrapper): Moved to bits/stl_function.h.
	* testsuite/20_util/function_ref/dangling.cc: New test.
	* testsuite/20_util/function_ref/dangling_neg.cc: New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

0 files changed, 0 insertions, 0 deletions