[Bug c/123362] Invalid conversions from nullptr generate misleading diagnostics

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123362

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The error message comes from convert_to_integer_1, convert_to_complex_1, and
convert_to_real_1.

Just need to add a case for NULLPTR_TYPE.