bug#34089: 27.0.50; Substring icomplete broken for M-x switch-to-buffer

bug#34089: 27.0.50; Substring icomplete broken for M-x switch-to-buffer

[João Távora]

Hi maintainers,

In my newfound icomplete adventures, I've come across a very annoying
bug I just cannot catch:

    Emacs -Q
    M-x icomplete-mode
    M-: (setq completion-styles '(substring)) RET
    M-: (generate-new-buffer "Messages") RET
    C-x b

Now type "M".  The minibuffer suggest the buffer "Messages", but what
about "*Messages"?  Why isn't it considered?

If, instead of "M", I type "essa", then "Messages" and "*Messages*"
_are_ both considered, correctly.

Tracing completion-substring-try/all-completion shows that it is not
being called in the first situation: why???

João

bug#34089: 27.0.50; Substring icomplete broken for M-x switch-to-buffer

[Stefan Monnier]

> Now type "M".  The minibuffer suggest the buffer "Messages", but what
> about "*Messages"?  Why isn't it considered?

Your (setq completion-styles '(substring)) is ignored by `C-x b`
because that command uses category `buffer` which has
a setting in completion-category-defaults of (styles basic substring),
so before trying `substring` it tries `basic`.

IOW, by default C-x b first tries prefix match and when that fails falls
back on `substring` and if you want to use `substring` without using
`basic` before, you'll need

    (add-to-list 'completion-category-overrides '(buffer (styles substring)))

Arguably this is a bug: the purpose of this completion-category-defaults
settings is to add substring after the global default `basic` style, so
if the global default starts with `substring` the better behavior would
be to just use the global default.

Not sure how to write the code that combines category-default-styles and
global styles in order to get that behavior.


        Stefan

bug#34089: 27.0.50; Substring icomplete broken for M-x switch-to-buffer

[João Távora]

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On Wed, Jan 16, 2019, 02:04 Stefan Monnier <monnier@iro.umontreal.ca wrote:

> Not sure how to write the code that combines category-default-styles and
> global styles in order to get that behavior.


Delete everything and try again?

I kid, I kid... :-) Thanks Stefan, I was hoping for such an easy answer!

BTW how do you debug this stuff?  Edebug is akward because of the
minibuffer usage (and the second frame technique doesn't always work).

João

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