`WARN` allows all types.

[python-processing-unit]

According to the specification section 9.1.12:

• BOOL: WARN(~STR: arg1, ..., ~STR: argN) = MUST behave like PRINT, except that output MUST occur only when the interpreter was started with -verbose, the rendered line MUST be prefixed with WARNING: , and the operator MUST return TRUE when output occurred and FALSE otherwise. Arguments outside INT and STR MUST raise a runtime error.

The implementation, however, allows any type as WARN args, not only those that can coerce to STR.