[Bug c++/88092] class nontype template deduction failed when providing type to class

[hanicka at hanicka dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88092

--- Comment #4 from Hana Dusíková <hanicka at hanicka dot net> ---
(In reply to Emmanuel Le Trong from comment #3)
> I have a problem with this. AFAIK, nowhere in the standard is written that a
> class template without argument (i.e. not a type!) can be used as the type
> of a non-type template argument. [temp.param] §4 says
> 
> > A non-type template-parameter shall have one of the following (optionally 
> > cv-qualified) types:
> >  — a type that is literal, has strong structural equality (10.10.1), has no 
> > mutable or volatile subobjects, and in which if there is a defaulted member 
> > operator<=>, then it is declared public,
> >  — an lvalue reference type,
> >  — a type that contains a placeholder type (9.1.7.5), or
> >  — a placeholder for a deduced class type (9.1.7.6).
> 
> So 
> 
>     template <class T> struct A {};
>     template <A a>     struct B {};
> 
> should not be valid IMO.

You are correct, you must have class template argument deduction rule. This
should be valid:

template <class T> struct A {
    constexpr A(...) {}
};
template <class T> A(T) -> A<T>;

template <A a> struct B {};