Subject: bug#40972: 27.0.91; assoc TESTFN's args reversed?

From: Mattias Engdegård <[email-masked]>
Date: Wed, 9 Sep 2020 16:46:43 +0200

> As Michael points out, the equality operation is symmetric and therefore by definition the order does not matter. We therefore cannot say if X=Y or Y=X is "more correct": they are equivalent.
> 
> In any case, I think it will cause more problems to switch the argument order at this point, since there might be code out there that relies on this (undocumented) behavior. So I'm closing this bug. 

It's quite possible that the current argument order was intended but never really documented. It's occasionally exploited (see image-converter--filter-formats). The asymmetry is stated more explicitly in the doc string and manual entry for assoc-default. Closing the bug is probably wisest.