On Tue, Jun 8, 2021 at 3:34 AM Eric Suen via Std-Discussion <std-discussion@lists.isocpp.org> wrote:
That's why I call it weird bug coz it's obvious. MSVC won't report any
error at that line with/without the comment.

Apparently MSVC allows any expression which looks like an unqualified template-id in a default template argument, even with "/permissive-" set. Being in a class doesn't even matter. If the default template argument is never used, the identifier assumed to be a template-name doesn't even need to appear anywhere else in the program. So it accepts things like:

template <class = A<void>::type, int = B<1>.M> void f();

This isn't exactly "weird bugs of compilers", though - MSVC is the only compiler with this issue.