A few questions:
* have you checked the provided lsp*.log files which show that the diagnostics is sent?
* have you checked the provided screenshots showing an exclamation mark in the fringe but no underlining?
* How do you explain the exclamation mark in the fringe which doess not appear if eglot is not enabled?
* Do you also see the exclamation mark in the fringe on the line in question?
* Does "cabal run" works? Do you see the output? If not please provide the complete error message.

If cabal is not working properly then you'll not see any diagnostics because the `bug73500.cabal` file includes a compiler directive to display all warnings.

Please provide the full `eglot-events-buffer` transcript.

Two `xx` still don't cause underlining.

On 29/09/2024 23:18, João Távora wrote:

On Sun, Sep 29, 2024 at 7:59 PM Federico Beffa
<federico.beffa@fbengineering.ch> wrote:

Sure, no problem and nothing changes, the bug is still there.

So far, there is one sighting of it: yours.  If I were you I would try
out the modified Main.hs program and report if you see the underlined
second x.

If you didn't know, Eglot has virtually 0 haskell-specific code (much
as it has 0 foo-specific code for any language foo).

João