minibuffer-completion-table as a hash table?

[Drew Adams]

Variable `minibuffer-completion-table' is an internal variable used in Emacs
completion operations. It can take several forms, such as an alist, an
obarray (vector), a list of strings, and a hash table (it can also be a
function).

Can someone tell me when (where) `minibuffer-completion-table' takes the
form of a hash table for Emacs completion? That is, besides passing
`all-completions' or `try-completion' a hash table of my own, where is a
hash table used in vanilla Emacs (for completion)? Are obarrays sometimes
implemented as hash tables, for example, or do they always take the form of
vectors?

In sum, if I grab `minibuffer-completion-table' at random during a
completion operation, under what circumstances is it likely to be a hash
table?  Thanks.