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

[Mattias Engdegård]

> 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.