bug#10416: 24.0.92; #'test-completion is inconsistent with symbol hash keys in COLLECTION

[Aidan Kehoe <kehoea@parhasard.net>]

 Ar an chéad lá de mí Eanair, scríobh Stefan Monnier: 

 > >  (try-completion "del-alist" #s(hash-table size 145 test equal rehash-size 1.5 rehash-threshold 0.8 data (del-alist hello "delay-mode-hooks" everyone "delete" everyone)))
 > >  => t ;; as expected and documented
 > 
 > >  (all-completions "del-alist" #s(hash-table size 145 test equal rehash-size 1.5 rehash-threshold 0.8 data (del-alist hello "delay-mode-hooks" everyone "delete" everyone)))
 > >  => ("del-alist") ;; as expected and documented
 > 
 > These are bugs.  The Elisp manual is pretty clear about it:
 > 
 >      If COLLECTION is a hash table, then the keys that are strings are
 >      the possible completions.  Other keys are ignored.

Also, throwing an error on encountering a symbol key (as #'test-completion
does) is not what that paragraph describes.

-- 
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described as “an unplanned and accidental public health triumph”, iodine was
added to cattle feed to improve milk production in the 1930s.’
(EN Pearce, Lancet, June 2011)