[Bug c++/67594] Bug on partial specialization? Incomplete type.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67594

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The primary template defines the default argument for the second parameter.

The C<float,I> partial specialization is irrelevant, it isn't used.

The C<float> specialization is identical to C<float, 1> because it uses the
default argument, so if you write it like that then it should be obvious the
code is wrong:

template <typename B, int I=1> class C{};

template <int I> class C<float,I>{};   // unused

template <> class C<float, 1> : public C<float, 1> {};
                  ^^^^^^^^^^^          ^^^^^^^^^^^

The base class matches the C<float, 1> specialization not the C<float, I>
partial specialization because C<float, 1> is more specialized.

So the base class is the same type, and of course a type cannot derive from
itself.