completion-regexp-list is not respected

[jdtsmith]

• Server used: python-lsp-server
• Emacs version: GNU Emacs 27.2
• Operating system: MacOS
• Eglot version: 20210901.917
• Eglot installation method: package-install
• Using Doom: No

The basic completion docs for try-completion (also applying to test-completion, all-completions, etc.) state (emphasis added):

In addition, to be acceptable, a completion must also match all the
regular expressions in ‘completion-regexp-list’. (Unless
COLLECTION is a function, in which case that function has to handle
‘completion-regexp-list’ itself.)

The COLLECTION function provided by eglot does not appear to consider completion-regexp-list. This variable is important for completion styles such as orderless, which put completion-regexp-list to heavy work for candidate refinement. Since most in-built completion tables are obarrays, alists, hashes, etc., the issue really only pertains to programmed completion, such as in eglot. I have a small patch to the proxies lambda which implements this, and works well in my testing. Let me know if you'd like to see a PR. See also oantolin/orderless#78. Thanks for eglot!

[nemethf]

In Emacs version 26.1, the docstring at the end of try-completion says: Additionally to this predicate, ‘completion-regexp-list’
is used to further constrain the set of candidates. To me, this means that try-completion internally takes care of filtering candidates according to completion-regexp-list. However, Eglot doesn't use all-completion. Isn't that the root cause of this issue?

I'm guessing here, but appropriately changing cl-remove-if-not to all-completion after the "all-completion" comment might just fix the issue.