The program is ill-formed in C++20 and onwards as explained here in this
stackoverflow question's answer
<https://stackoverflow.com/questions/71972000/is-having-a-declaration-stackt-for-the-default-ctor-valid-inside-a-class-te>.
In the above you'll find answer for pre and post C++20. It's just that not
all compilers have implemented/applied this change yet. Demo
<https://godbolt.org/z/7dqMhYaxd>. Only gcc correctly rejects this in C++20
while and msvc and clang both accepts this and looks like they have not yet
made this change in their implementation.

On Thu, 18 Jan 2024 at 13:14, Stephan Bergmann via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> My naive understanding is that
>
> > emplate<typename T> struct S { S<T>(); };
>
> is not well-formed: [class.ctor.general]/1.2 requires that:
>
> "otherwise, in a member-declaration that belongs to the
> member-specification of a class or class template, the id-expression is
> the injected-class-name (11.1) of the immediately-enclosing entity;"
>
> (which is the case here), and [class.pre]/2 defines that:
>
> "The class-name is also bound in the scope of the class (template)
> itself; this is known as the injected-class-name."
>
> and [class.pre]/1 requires that:
>
> "A class declaration where the class-name in the class-head-name is a
> simple-template-id shall be an explicit specialization (13.9.4) or a
> partial specialization (13.7.6)."
>
> (which is not the case here).
>
> Yet, what puzzles me is that compilers generally seem to accept the
> above (see <https://godbolt.org/z/P83Psv7nx>, where the failure produced
> by GCC -std=c++20 looks more like it mis-parses this than that it
> genuinely wants to reject this as not well-formed). What am I missing?
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>