bug#50583: 27.2; test-completion should return non-nil

[Dmitry Gutov]

On 14.09.2021 16:25, Kevin Vigouroux via Bug reports for GNU Emacs, the 
Swiss army knife of text editors wrote:
> In the following code, I assume that `test-completion' should return a
> non-‘nil’ value because "foo" is a valid completion alternative.

"foo" is not a member of the list '("foobar" "bar" "barfoo"), so that's 
not true.

Perhaps you want to use 'try-completion' instead?