[Git][ghc/ghc][wip/T22623] Fix an assertion check in addToEqualCtList
Simon Peyton Jones (@simonpj)
gitlab at gitlab.haskell.org
Tue Dec 20 00:08:33 UTC 2022
Simon Peyton Jones pushed to branch wip/T22623 at Glasgow Haskell Compiler / GHC
Commits:
e3289d7e by Simon Peyton Jones at 2022-12-19T23:26:06+00:00
Fix an assertion check in addToEqualCtList
The old assertion saw that a constraint ct could rewrite itself
(of course it can) and complained (stupid).
- - - - -
4 changed files:
- compiler/GHC/Tc/Solver/Types.hs
- + testsuite/tests/typecheck/should_fail/T22645.hs
- + testsuite/tests/typecheck/should_fail/T22645.stderr
- testsuite/tests/typecheck/should_fail/all.T
Changes:
=====================================
compiler/GHC/Tc/Solver/Types.hs
=====================================
@@ -273,21 +273,29 @@ addToEqualCtList ct old_eqs
| debugIsOn
= case ct of
CEqCan { cc_lhs = TyVarLHS tv } ->
- let shares_lhs (CEqCan { cc_lhs = TyVarLHS old_tv }) = tv == old_tv
- shares_lhs _other = False
- in
- assert (all shares_lhs old_eqs) $
- assert (null ([ (ct1, ct2) | ct1 <- ct : old_eqs
- , ct2 <- ct : old_eqs
- , let { fr1 = ctFlavourRole ct1
- ; fr2 = ctFlavourRole ct2 }
- , fr1 `eqCanRewriteFR` fr2 ])) $
+ assert (all (shares_lhs tv) old_eqs) $
+ assertPpr (null bad_prs)
+ (vcat [ text "bad_prs" <+> ppr bad_prs
+ , text "ct:old_eqs" <+> ppr (ct : old_eqs) ]) $
(ct : old_eqs)
_ -> pprPanic "addToEqualCtList not CEqCan" (ppr ct)
| otherwise
= ct : old_eqs
+ where
+ shares_lhs tv (CEqCan { cc_lhs = TyVarLHS old_tv }) = tv == old_tv
+ shares_lhs _ _ = False
+ bad_prs = filter is_bad_pair (distinctPairs (ct : old_eqs))
+ is_bad_pair (ct1,ct2) = ctFlavourRole ct1 `eqCanRewriteFR` ctFlavourRole ct2
+
+distinctPairs :: [a] -> [(a,a)]
+-- distinctPairs [x1,...xn] is the list of all pairs [ ...(xi, xj)...]
+-- where i /= j
+-- NB: does not return pairs (xi,xi), which would be stupid in the
+-- context of addToEqualCtList (#22645)
+distinctPairs [] = []
+distinctPairs (x:xs) = concatMap (\y -> [(x,y),(y,x)]) xs ++ distinctPairs xs
-- returns Nothing when the new list is empty, to keep the environments smaller
filterEqualCtList :: (Ct -> Bool) -> EqualCtList -> Maybe EqualCtList
=====================================
testsuite/tests/typecheck/should_fail/T22645.hs
=====================================
@@ -0,0 +1,9 @@
+module T22645 where
+
+import Data.Coerce
+
+type T :: (* -> *) -> * -> *
+data T m a = MkT (m a)
+
+p :: Coercible a b => T Maybe a -> T Maybe b
+p = coerce
=====================================
testsuite/tests/typecheck/should_fail/T22645.stderr
=====================================
@@ -0,0 +1,17 @@
+T22645.hs:9:5: error: [GHC-25897]
+ • Couldn't match type ‘a’ with ‘b’ arising from a use of ‘coerce’
+ ‘a’ is a rigid type variable bound by
+ the type signature for:
+ p :: forall a b. Coercible a b => T Maybe a -> T Maybe b
+ at T22645.hs:8:1-44
+ ‘b’ is a rigid type variable bound by
+ the type signature for:
+ p :: forall a b. Coercible a b => T Maybe a -> T Maybe b
+ at T22645.hs:8:1-44
+ • In the expression: coerce
+ In an equation for ‘p’: p = coerce
+ • Relevant bindings include
+ p :: T Maybe a -> T Maybe b (bound at T22645.hs:9:1)
+ |
+9 | p = coerce
+ | ^^^^^^
=====================================
testsuite/tests/typecheck/should_fail/all.T
=====================================
@@ -666,3 +666,4 @@ test('T21447', normal, compile_fail, [''])
test('T21530a', normal, compile_fail, [''])
test('T21530b', normal, compile_fail, [''])
test('T22570', normal, compile_fail, [''])
+test('T22645', normal, compile_fail, [''])
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/e3289d7e1e554b3c54dd605125b63930b606dd5a
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/e3289d7e1e554b3c54dd605125b63930b606dd5a
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