poor diagnostic for unused function parameter

[rickycodes]

got this warning:

and then following the diagnostic suggestion it broke:

using .. suppresses the warning and works as intended, so perhaps that's what should be suggested?

this was done using notify's raw_watcher

[zackmdavis]

(A duplicate of #47390, this is being fixed in #47922, after which the suggestion would be cookie: _.)

[TimNN]

Closing as duplicate.