Reinstate untouchable variable error messages

This extra bit of information was accidentally being discarded after a refactoring of the way we reported problems when unifying a type variable with another type. See #23591 (closed)

This patch rectifies that.

TODO:

• accept testsuite changes.

added backport needed:9.6 backport needed:9.8 labels

added 1 commit

• b0dc18a9 - Reinstate untouchable variable error messages

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mentioned in issue #23591 (closed)

@mpickering: could I ask to squeeze this into 9.6.3?

Yes I don't see why not.

changed the description

Better!

Does this give a more helpul message in Mikolaj's example?

It would be good to have a regression test. Currently we have none. Sadly, the simple ones I describe in #23591 (closed) do not cough up an untouchable error because the untouchable variable is skolemised. But that's not the fault of your patch

added 1 commit

• 8c1e1724 - Reinstate untouchable variable error messages

Compare with previous version

marked the checklist item accept testsuite changes. as completed

marked this merge request as ready

mentioned in issue #23589

assigned to @marge-bot

I will attempt to batch this MR (!10812 (closed))...

Merged in 2b55cb5f

closed

mentioned in merge request !10924 (closed)

Backported to %9.8.1 with d33581c5.

changed milestone to %9.8.1

removed backport needed:9.8 label

mentioned in issue #23590 (closed)

mentioned in merge request !11203 (merged)

Backported to %9.6.3 in !11203 (merged)

removed backport needed:9.6 label