[Bug c++/80955] Macros expanded in definition of user-defined literals

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80955

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Is there a way to check if it's a pre-defined macro?

Or just change the condition to allow starting with two underscores, but not
allow starting with one (since user-defined UDLs must start with a single
underscore, and must not start with two).