https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86193
Jason Merrill <jason at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |SUSPENDED
--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> (In reply to zhonghao from comment #0)
> > I tried the latest g++, but it does not accept the code. BTW, clang++
> > accepts the code. Is this a recurring bug?
>
> No, it's rejected for a different reason, so it's not the same bug.
>
> It started to be rejected with r243868:
>
> Check that a partial specialization is more specialized.
Indeed.
[temp.class.spec]: The specialization shall be more specialized than the
primary template (17.6.5.2).
Checking that is equivalent to resolving the partial ordering of function
templates in
template <class T>
struct identity
{
typedef T type;
};
template <class T, typename T::type A>
struct foo {};
template <class T1, typename T1::type A1>
void fn(foo<T1,A1>) = delete;
template <class T2, T2 A2>
void fn(foo<identity<T2>,A2>);
int main()
{
fn(foo<identity<int>,0>()); // error here
}
which, similarly, G++ (and EDG) rejects, and clang accepts.
I think G++ is right here: when we try to deduce values for T1 and A1 in
partial ordering, we deduce T1 to identity<T2>, but we can't deduce anything
for A1 because we don't know that the types are compatible. You can work
around that by changing your partial specialization to
template <class T, typename identity<T>::type A>
struct foo<identity<T>, A> {};
This does seem like an underspecified area in the standard.
