[Git][ghc/ghc][master] Buglet in GHC.Tc.Module.checkBootTyCon

[Marge Bot (@marge-bot)]

Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
ef511b23 by Simon Peyton Jones at 2022-11-20T18:40:05-05:00
Buglet in GHC.Tc.Module.checkBootTyCon

This lurking bug used the wrong function to compare two
types in GHC.Tc.Module.checkBootTyCon

It's hard to trigger the bug, which only came up during
!9343, so there's no regression test in this MR.

- - - - -


1 changed file:

- compiler/GHC/Tc/Module.hs


Changes:

=====================================
compiler/GHC/Tc/Module.hs
=====================================
@@ -1096,6 +1096,7 @@ checkBootTyCon is_boot tc1 tc2
        -- Order of pattern matching matters.
        subDM _ Nothing _ = True
        subDM _ _ Nothing = False
+
        -- If the hsig wrote:
        --
        --   f :: a -> a
@@ -1103,11 +1104,14 @@ checkBootTyCon is_boot tc1 tc2
        --
        -- this should be validly implementable using an old-fashioned
        -- vanilla default method.
-       subDM t1 (Just (_, GenericDM t2)) (Just (_, VanillaDM))
-        = eqTypeX env t1 t2
+       subDM t1 (Just (_, GenericDM gdm_t1)) (Just (_, VanillaDM))
+        = eqType t1 gdm_t1   -- Take care (#22476).  Both t1 and gdm_t1 come
+                             -- from tc1, so use eqType, and /not/ eqTypeX
+
        -- This case can occur when merging signatures
        subDM t1 (Just (_, VanillaDM)) (Just (_, GenericDM t2))
         = eqTypeX env t1 t2
+
        subDM _ (Just (_, VanillaDM)) (Just (_, VanillaDM)) = True
        subDM _ (Just (_, GenericDM t1)) (Just (_, GenericDM t2))
         = eqTypeX env t1 t2



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/ef511b235f10101f9923e3f098eeeb6a03a734d8

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/ef511b235f10101f9923e3f098eeeb6a03a734d8
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