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Patrick pushed to branch wip/soulomoon/T26154-work-around at Glasgow Haskell Compiler / GHC

Commits:

04ed1efa

by soulomoon at 2025-07-02T22:12:15+08:00

Fix crash in family instance consistency checking

Modules from ModIface Dependencies and home package family instance
environment (built from module graph) are not always consistent.
Use empty family instance environment when module is not found
instead of crashing during checkFamInstConsistency.

Fix #26154

1 changed file:

compiler/GHC/Tc/Instance/Family.hs

Changes:

compiler/GHC/Tc/Instance/Family.hs

@@ -405,7 +405,13 @@ getFamInsts hpt_fam_insts mod
    | Just env <- lookupModuleEnv hpt_fam_insts mod = return env
    | otherwise = do { _ <- initIfaceTcRn (loadSysInterface doc mod)
                     ; eps <- getEps
 -                   ; return (expectJust $
 +                   -- Workaround for #26154:
 +                   -- We use `fromMaybe emptyFamInstEnv` instead of `expectJust` because
 +                   -- the module `mod` comes from the `ModIface`'s `Dependencies`, while
 +                   -- `hpt_fam_insts` is built from `moduleGraphModulesBelow`.
 +                   -- Due to inconsistencies between the two, `mod` might not be present
 +                   -- in `hpt_fam_insts`.
 +                   ; return (fromMaybe emptyFamInstEnv $
                               lookupModuleEnv (eps_mod_fam_inst_env eps) mod) }
    where
      doc = ppr mod <+> text "is a family-instance module"