4c2ffb02fd
This PR points out that std::is_convertible has given the wrong answer
in
static_assert (!std::is_convertible_v <int () const, int (*) ()>, "");
since r13-2822 implemented __is_{,nothrow_}convertible.
std::is_convertible uses the imaginary
To test() { return std::declval<From>(); }
to do its job. Here, From is 'int () const'. std::declval is defined as:
template<class T>
typename std::add_rvalue_reference<T>::type declval() noexcept;
std::add_rvalue_reference is defined as "If T is a function type that
has no cv- or ref- qualifier or an object type, provides a member typedef
type which is T&&, otherwise type is T."
In our case, T is cv-qualified, so the result is T, so we end up with
int () const declval() noexcept;
which is invalid. In other words, this is pretty much like:
using T = int () const;
T fn1(); // bad, fn returning a fn
T& fn2(); // bad, cannot declare reference to qualified function type
T* fn3(); // bad, cannot declare pointer to qualified function type
using U = int ();
U fn4(); // bad, fn returning a fn
U& fn5(); // OK
U* fn6(); // OK
I think is_convertible_helper needs to simulate std::declval better.
To that end, I'm introducing build_trait_object, to be used where
a declval is needed.
PR c++/109680
gcc/cp/ChangeLog:
* method.cc (build_trait_object): New.
(assignable_expr): Use it.
(ref_xes_from_temporary): Likewise.
(is_convertible_helper): Likewise. Check FUNC_OR_METHOD_TYPE_P.
gcc/testsuite/ChangeLog:
* g++.dg/ext/is_convertible6.C: New test.
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