Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC Commits: 41e80f41 by Simon Peyton Jones at 2026-10-11T14:38:51-04:00 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. - - - - - 53 changed files: - + changelog.d/T27731 - compiler/GHC/Builtin/WiredIn/Ids.hs - compiler/GHC/Rename/Module.hs - compiler/GHC/Tc/Errors.hs - compiler/GHC/Tc/Errors/Ppr.hs - compiler/GHC/Tc/Errors/Types.hs - compiler/GHC/Tc/Instance/Family.hs - compiler/GHC/Tc/Solver/Default.hs - compiler/GHC/Tc/Solver/Equality.hs - compiler/GHC/Tc/Types/Constraint.hs - compiler/GHC/Tc/Utils/Monad.hs - testsuite/tests/ghci/prog020/ghci.prog020a.stderr - testsuite/tests/ghci/prog020/ghci.prog020b.stderr - testsuite/tests/ghci/prog021/prog021a.stderr - testsuite/tests/ghci/prog021/prog021b.stderr - testsuite/tests/ghci/prog022/ghci.prog022a.stderr - testsuite/tests/ghci/prog022/ghci.prog022b.stderr - testsuite/tests/ghci/prog022/ghci.prog022c.stderr - testsuite/tests/ghci/prog022/ghci.prog022d.stderr - testsuite/tests/ghci/prog022/ghci.prog022e.stderr - testsuite/tests/ghci/prog022/ghci.prog022f.stderr - testsuite/tests/ghci/scripts/Defer02.stderr - testsuite/tests/module/mod130.stderr - testsuite/tests/overloadedstrings/should_fail/T25926.stderr - testsuite/tests/overloadedstrings/should_fail/T27124.stderr - testsuite/tests/patsyn/should_fail/records-nofieldselectors.stderr - testsuite/tests/printer/T17697.stderr - testsuite/tests/rename/should_fail/T15607.stderr - testsuite/tests/rename/should_fail/T19843b.stderr - testsuite/tests/safeHaskell/ghci/p4.stderr - testsuite/tests/safeHaskell/ghci/p6.stderr - testsuite/tests/typecheck/should_compile/T11254.stderr - testsuite/tests/typecheck/should_compile/T15370.stderr - testsuite/tests/typecheck/should_compile/valid_hole_fits.stderr - testsuite/tests/typecheck/should_fail/ClassOperator.stderr - testsuite/tests/typecheck/should_fail/DoExpansion2.stderr - testsuite/tests/typecheck/should_fail/DoExpansion3.stderr - testsuite/tests/typecheck/should_fail/FunDepOrigin1b.hs - testsuite/tests/typecheck/should_fail/T18851.stderr - + testsuite/tests/typecheck/should_fail/T18851e.hs - + testsuite/tests/typecheck/should_fail/T18851e.stderr - + testsuite/tests/typecheck/should_fail/T18851f.hs - + testsuite/tests/typecheck/should_fail/T18851f.stderr - testsuite/tests/typecheck/should_fail/T22940.stderr - testsuite/tests/typecheck/should_fail/T26823.stderr - + testsuite/tests/typecheck/should_fail/T27731.hs - + testsuite/tests/typecheck/should_fail/T27731.stderr - testsuite/tests/typecheck/should_fail/T5691.stderr - testsuite/tests/typecheck/should_fail/T7437.stderr - testsuite/tests/typecheck/should_fail/all.T - testsuite/tests/typecheck/should_fail/mc24.stderr - testsuite/tests/typecheck/should_fail/tcfail019.stderr - testsuite/tests/typecheck/should_fail/tcfail189.stderr The diff was not included because it is too large. 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