[Git][ghc/ghc][master] Valid hole fits: don't panic on a Given

[Marge Bot (@marge-bot)]

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


Commits:
630e3026 by sheaf at 2023-07-12T11:26:43-04:00
Valid hole fits: don't panic on a Given

The function GHC.Tc.Errors.validHoleFits would end up panicking when
encountering a Given constraint. To fix this, it suffices to filter out
the Givens before continuing.

Fixes #22684

- - - - -


5 changed files:

- compiler/GHC/Tc/Errors.hs
- compiler/GHC/Tc/Errors/Types.hs
- + testsuite/tests/typecheck/should_fail/T22684.hs
- + testsuite/tests/typecheck/should_fail/T22684.stderr
- testsuite/tests/typecheck/should_fail/all.T


Changes:

=====================================
compiler/GHC/Tc/Errors.hs
=====================================
@@ -1563,16 +1563,19 @@ validHoleFits :: SolverReportErrCtxt    -- ^ The context we're in, i.e. the
                 -- the valid hole fits.
 validHoleFits ctxt@(CEC { cec_encl = implics
                         , cec_tidy = lcl_env}) simps hole
-  = do { (tidy_env, fits) <- findValidHoleFits lcl_env implics (map mk_wanted simps) hole
+  = do { (tidy_env, fits) <- findValidHoleFits lcl_env implics (mapMaybe mk_wanted simps) hole
        ; return (ctxt {cec_tidy = tidy_env}, fits) }
   where
-    mk_wanted :: ErrorItem -> CtEvidence
-    mk_wanted (EI { ei_pred = pred, ei_evdest = Just dest, ei_loc = loc })
-         = CtWanted { ctev_pred      = pred
-                    , ctev_dest      = dest
-                    , ctev_loc       = loc
-                    , ctev_rewriters = emptyRewriterSet }
-    mk_wanted item = pprPanic "validHoleFits no evdest" (ppr item)
+    mk_wanted :: ErrorItem -> Maybe CtEvidence
+    mk_wanted (EI { ei_pred = pred, ei_evdest = m_dest, ei_loc = loc })
+      | Just dest <- m_dest
+      = Just (CtWanted { ctev_pred      = pred
+                       , ctev_dest      = dest
+                       , ctev_loc       = loc
+                       , ctev_rewriters = emptyRewriterSet })
+      | otherwise
+      = Nothing   -- The ErrorItem was a Given
+
 
 -- See Note [Constraints include ...]
 givenConstraints :: SolverReportErrCtxt -> [(Type, RealSrcSpan)]


=====================================
compiler/GHC/Tc/Errors/Types.hs
=====================================
@@ -4867,7 +4867,9 @@ data ErrorItem
   = EI { ei_pred     :: PredType         -- report about this
          -- The ei_pred field will never be an unboxed equality with
          -- a (casted) tyvar on the right; this is guaranteed by the solver
-       , ei_evdest   :: Maybe TcEvDest   -- for Wanteds, where to put evidence
+       , ei_evdest   :: Maybe TcEvDest
+         -- ^ for Wanteds, where to put the evidence
+         --   for Givens, Nothing
        , ei_flavour  :: CtFlavour
        , ei_loc      :: CtLoc
        , ei_m_reason :: Maybe CtIrredReason  -- if this ErrorItem was made from a


=====================================
testsuite/tests/typecheck/should_fail/T22684.hs
=====================================
@@ -0,0 +1,19 @@
+module T22684 where
+
+-- Example 1 from #22684
+p :: (Int ~ Bool => r) -> r
+p _ = undefined
+
+q :: r
+q = p _
+
+-- Example 3 from #22684
+class Category k where
+  (.) :: k b c -> k a b -> k a c
+
+data Free p a b where
+  Prod :: Free p a (b, c)
+  Sum  :: Free p (Either a b) c
+
+instance Category (Free p) where
+  Sum . Prod = _


=====================================
testsuite/tests/typecheck/should_fail/T22684.stderr
=====================================
@@ -0,0 +1,35 @@
+
+T22684.hs:8:7: error: [GHC-88464]
+    • Found hole: _ :: r
+      Where: ‘r’ is a rigid type variable bound by
+               the type signature for:
+                 q :: forall r. r
+               at T22684.hs:7:1-6
+    • In the first argument of ‘p’, namely ‘_’
+      In the expression: p _
+      In an equation for ‘q’: q = p _
+    • Relevant bindings include q :: r (bound at T22684.hs:8:1)
+      Constraints include Int ~ Bool (from T22684.hs:8:7)
+      Valid hole fits include q :: r (bound at T22684.hs:8:1)
+
+T22684.hs:19:16: error: [GHC-88464]
+    • Found hole: _ :: Free p a c
+      Where: ‘k’, ‘p’ are rigid type variables bound by
+               the instance declaration
+               at T22684.hs:18:10-26
+             ‘a’, ‘c’ are rigid type variables bound by
+               the type signature for:
+                 (T22684..) :: forall b c a. Free p b c -> Free p a b -> Free p a c
+               at T22684.hs:19:7
+    • In an equation for ‘T22684..’: Sum T22684.. Prod = _
+      In the instance declaration for ‘Category (Free p)’
+    • Relevant bindings include
+        (.) :: Free p b c -> Free p a b -> Free p a c
+          (bound at T22684.hs:19:7)
+      Constraints include
+        b ~ (b2, c1) (from T22684.hs:19:9-12)
+        b ~ Either a1 b1 (from T22684.hs:19:3-5)
+      Valid hole fits include
+        q :: forall r. r
+          with q @(Free p a c)
+          (bound at T22684.hs:8:1)


=====================================
testsuite/tests/typecheck/should_fail/all.T
=====================================
@@ -696,4 +696,5 @@ test('VisFlag2', normal, compile_fail, [''])
 test('VisFlag3', normal, compile_fail, [''])
 test('VisFlag4', normal, compile_fail, [''])
 test('VisFlag5', normal, compile_fail, [''])
+test('T22684', normal, compile_fail, [''])
 test('T23514a', normal, compile_fail, [''])



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/630e302617a4a3e00d86d0650cb86fa9e6913e44

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/630e302617a4a3e00d86d0650cb86fa9e6913e44
You're receiving this email because of your account on gitlab.haskell.org.


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-commits/attachments/20230712/65a9063a/attachment-0001.html>