libstdc++: Make INVOKE<R> refuse to create dangling references [PR70692]

This is the next part of the library changes from P2255R2. This makes
INVOKE<R> ill-formed if converting the INVOKE expression to R would bind
a reference to a temporary object.

The is_invocable_r trait is now false if the invocation would create a
dangling reference. This is done by adding the dangling check to the
__is_invocable_impl partial specialization used for INVOKE<R>
expressions. This change also slightly simplifies the nothrow checking
recently added to that partial specialization.

This change also removes the is_invocable_r checks from the pre-C++17
implementation of std::__invoke_r, because there is no need for it to be
SFINAE-friendly. None of our C++11 and C++14 uses of INVOKE<R> require
those constraints. The std::function constructor needs to check
is_invocable_r, but that's already done explicitly, so we don't need to
recheck when calling __is_invoke_r in std::function::operator(). The
other uses of std::__is_invoke_r do not need to be constrained and can
just be ill-formed if the INVOKE<R> expression is ill-formed.


	PR libstdc++/70692
	* include/bits/invoke.h [__cplusplus < 201703] (__invoke_r):
	Remove is_invocable and is_convertible constraints.
	* include/std/type_traits (__is_invocable_impl::_S_conv): Use
	non-deduced context for parameter.
	(__is_invocable_impl::_S_test): Remove _Check_noex template
	parameter and use deduced noexcept value in its place. Add bool
	parameter to detect dangling references.
	(__is_invocable_impl::type): Adjust call to _S_test to avoid
	deducing unnecessary noexcept property..
	(__is_invocable_impl::__nothrow_type): Rename to ...
	(__is_invocable_impl::__nothrow_conv): ... this. Adjust call
	to _S_test to deduce noexcept property.
	* testsuite/20_util/bind/ New test.
	* testsuite/20_util/function/cons/ New test.
	* testsuite/20_util/function_objects/invoke/
	New test.
	* testsuite/20_util/is_invocable/ New test.
	* testsuite/30_threads/packaged_task/cons/
	New test.
7 files changed