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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.
file: libstdc++-v3/README New users may wish to point their web browsers to the file index.html in the 'doc/html' subdirectory. It contains brief building instructions and notes on how to configure the library in interesting ways.