Extension:ParserFun depends on Extension:Validator, but it expects it to be explicitly called from LocalSettings.php in the old style of loading extensions.

Modern extensions that depend on Validator do so via Composer, such as the case with SMW and others.

Explicitly invoking Validator from LocalSettings.php while it is already being loaded by another means causes breakage.

Currently it is not possible to use ParserFun if another extension is already loading Validator in the new style, and as such the

Extension:ParserFun needs to be updated in order to be able to use Extension:Validator.

Alternatively, implementing THIS and CALLER in core as described in T204856 could remove the need for ParserFun