I just ran into a bunch of new deprecations on master, it seems that isalnum has been deprecated, and it is suggested to use isalpha(c) || isnumeric(c) instead.
isalnum is very heavily used in some code, for example where identifiers are defined as starting with isalpha and then allowing any number of isalnum characters, and the change can have a large impact on performance (since isalnum would generally be implemented not by two lookups, but by a single lookup into a two-stage bit table indexed by code point, exactly the same as the other is* POSIX functions).
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