[Bug libstdc++/70692] No warning when std::function<const int&(...)> binds a reference to a temporary

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70692

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:fa9bda3ea4315a7285edbc99323e3fa7885cbbb8

commit r13-2922-gfa9bda3ea4315a7285edbc99323e3fa7885cbbb8
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Sep 27 20:59:05 2022 +0100

    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.

    libstdc++-v3/ChangeLog:

            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/dangling_ref.cc: New test.
            * testsuite/20_util/function/cons/70692.cc: New test.
            * testsuite/20_util/function_objects/invoke/dangling_ref.cc:
            New test.
            * testsuite/20_util/is_invocable/dangling_ref.cc: New test.
            * testsuite/30_threads/packaged_task/cons/dangling_ref.cc:
            New test.