Permanent "not indexed" error with Clang 12 and above

Observed behavior

Everything works great when I compile ccls against versions of Clang up to and including 11.1.0.

When I compile ccls against anything newer (well, tested with 12.0 and 14.0), and no other changes at all just a different clang, then I get nothing: all I see in my LSP client is "not indexed". LSP mode shows in the minibuffer:

LSP :: Error from the Language Server: not indexed (Invalid Request)

I've enabled --log-file=ccls.out -v=9 and looked at the log file and there is absolutely nothing odd there: no error messages or warnings or anything.

I've enabled client logging on the Emacs LSP mode and all I ever get back from ccls is:

[Trace - 02:03:46 PM] Received response 'nil - (22)' in 999ms.
Result: {
  "message": "not indexed",
  "code": -32600
}

[Trace - 02:03:46 PM] Received response 'textDocument/documentHighlight - (23)' in 31ms.
Result: {
  "message": "not indexed",
  "code": -32600
}

It appears that indexing never proceeds or succeeds, but there's absolutely nothing that I can find in any logging that would explain why or how to fix it.

Also, no files are ever created in the .ccls-cache directory!

Steps to reproduce

I doubt it's reproducible outside my environment, unless others also have problems using Clang 12+.

What I really need is some kind of pointer that allows me to get more debugging or figure out why the indexing just never seems to do anything. Is there a way to compile ccls with more debugging output?

System information

ccls version: 0.20210330-11-g790daca4
clang version: clang 12.0, clang 14.0
OS: GNU/Linux
Editor: Emacs 27.2
Language client (and version): Emacs lsp-mode 20220401.1003 (but have tried various) and ccls-20200820.308

MaskRay / ccls