https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103110
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I think this is a bug in clang as GCC, ICC (EDG) and MSVC all reject the code
for the same reason.
GCC even warns about it:
<source>:3:14: warning: use of 'auto' in member template conversion operator
can never be deduced
3 | operator auto() const { return 2; }
| ^~~~
