[Bug c++/72842] non-type template-parameter of type void

[wjwray at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72842

Will Wray <wjwray at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |wjwray at gmail dot com

--- Comment #8 from Will Wray <wjwray at gmail dot com> ---
Reporting a (non-variadic) case here in which gcc incorrectly accepts void,
and now also accepts class types not satisfying the requirements for a
class type non-type template parameter (now they are implemented on trunk).

https://godbolt.org/z/fjFzMm

    template <typename T,T> struct TT;
    template <typename T, template <typename,T> class TT> struct TTT {};

    TTT<void,TT> x;

GCC accepts the variable declaration
(or explicit instantiation    template struct TTT<void,TT>; )

Clang rejects with:

    error: a non-type template parameter cannot have type 'void'
    template <typename T, template <typename,T> class TT> struct TTT {};
                                             ^
This is easy to CONFIRM via the compiler explorer link above.

Also
With class-type non-type template parameters now in gcc trunk
GCC incorrectly accepts this:

    struct NotNTTP { int mutable i; };

    TTT<NotNTTP,TT> c;

https://godbolt.org/z/jfioyB

(The compiler explorer link implements a constraint to test
 if a type fits the requirements of a non-type template parameter,
 otherwise hard to implement without relying on default construction)

Perhaps this should be a new bug as the title is specific to void.