a9864e81
by Simon Peyton Jones at 2026-10-09T17:21:11+01:00
Simplify the error-suppression mechanism in GHC.Tc.Errors
In `GHC.Tc.Errors.reportWanteds` we suppress some errors in favour of
others. But the mechanism had grown crufty, and #27331 exposed a bug.
This MR makes it simpler and more uniform, by doing everything via
the `ei_suppress` field of the ErrorItem.
Claude then found another closely-related bug (see the review linked
on !16564). That bug is immortalised in new test T18851d.
I'm still not happy with `ignoreConstraint` but we can worry about
that another day; I have not touched it.
Main changes
* Instead of /predicting/ when an error will be reported, we set
`cec_error_reported` when an error is /actually/ reported. (I renamed
it from `cec_suppress` to reflect this change.)
Now errors are suppressed only when a preceding error has actually been
reported. Simpler and more robust.
See `Note [cec_error_reported: suppressing less-serious errors]`
The big changes are in `reportWanteds` which looks very different.
* New `Note [ei_suppress: suppressing confusing errors]` describes the
(quite separate) `ei_suppress` mechanism for suppressing confusing
errors, mainly arising from non-empty rewriter sets
* In `tryTypeClassDefaulting`, instead of switching off type-class defaulting
if there is an insoluble /anywhere/, we just switch it off locally when
we find an insoluble sub-tree. See Note [When to do type-class defaulting]
and (W5) in Note [ApproximateWC]
* Greatly simplify `insolubleWantedCt`.
* I made type-class-defaulting be a bit more eager, only switched off when
a peer or ancestor (but not child) constraint is insoluble. See
Note [When to do type-class defaulting].
Small refactorings
* Rename `askNoErrs` to `askErrsFound` and flip its polarity
* The code for "check if ei_suppress is true of all items" code is
moved to `mkErrorItems` rather than in the large `reportWanteds`.
* Code for `has_gadt_match` moves to `mkGivenErrorReporter`, rather than
polluting the large `reportWanteds`.
* Many tests with `-fdefer-type-errors` get more warnings, especially
about ambiguous (Num a) constraints. This is by-design: see
Note [Always warn with -fdefer-type-errors]. Previously GHC was not
honouring this Note but now it is, hence the extra errors.