f[x] ^= 1; ValueQ[g[x]]
True in Mathematica 12.2. Is it a bug?
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The behavior of
ValueQ has changed in version 12.2. In fact, you can provide a
Method -> "Legacy" option to match the behavior the function had in Version 12.1 and earlier:
The Details portion of the documentation for
ValueQ now states that the default method in the 12.2 implementation (
Automatic) effectively uses
expr is a symbol and
"SymbolDefinitionsPresent" otherwise (which means that "one or more symbols have any sort of definition present"). As I understand it,
g[x] is not a symbol, so the latter is used.
This change for version 12.2 is also mentioned in the list of incompatible changes between MMA versions.