[Git][ghc/ghc][wip/T22948] Treat type data declarations as empty when checking pattern-matching coverage

[Ryan Scott (@RyanGlScott)]

Ryan Scott pushed to branch wip/T22948 at Glasgow Haskell Compiler / GHC


Commits:
78dc538f by Ryan Scott at 2023-02-11T18:35:05-05:00
Treat type data declarations as empty when checking pattern-matching coverage

The data constructors for a `type data` declaration don't exist at the value
level, so we don't want GHC to warn users to match on them.

Fixes #22964.

- - - - -


4 changed files:

- compiler/GHC/HsToCore/Pmc/Solver.hs
- compiler/GHC/Rename/Module.hs
- + testsuite/tests/pmcheck/should_compile/T22964.hs
- testsuite/tests/pmcheck/should_compile/all.T


Changes:

=====================================
compiler/GHC/HsToCore/Pmc/Solver.hs
=====================================
@@ -146,11 +146,16 @@ updRcm f (RCM vanilla pragmas)
 -- Ex.: @vanillaCompleteMatchTC 'Maybe' ==> Just ("Maybe", {'Just','Nothing'})@
 vanillaCompleteMatchTC :: TyCon -> Maybe CompleteMatch
 vanillaCompleteMatchTC tc =
-  let -- TYPE acts like an empty data type on the term-level (#14086), but
-      -- it is a PrimTyCon, so tyConDataCons_maybe returns Nothing. Hence a
-      -- special case.
-      mb_dcs | tc == tYPETyCon = Just []
-             | otherwise       = tyConDataCons_maybe tc
+  let mb_dcs | -- TYPE acts like an empty data type on the term level (#14086),
+               -- but it is a PrimTyCon, so tyConDataCons_maybe returns Nothing.
+               -- Hence a special case.
+               tc == tYPETyCon    = Just []
+             | -- Similarly, treat `type data` declarations as empty data types on
+               -- the term level, as `type data` data constructors only exist at
+               -- the type level (#22964).
+               -- See Note [Type data declarations] in GHC.Rename.Module.
+               isTypeDataTyCon tc = Just []
+             | otherwise          = tyConDataCons_maybe tc
   in vanillaCompleteMatch . mkUniqDSet . map RealDataCon <$> mb_dcs
 
 -- | Initialise from 'dsGetCompleteMatches' (containing all COMPLETE pragmas)


=====================================
compiler/GHC/Rename/Module.hs
=====================================
@@ -2146,6 +2146,21 @@ The main parts of the implementation are:
   declared as a `type data` declaration, however, the wrapper is omitted.
   See `wrapped_reqd` in GHC.Types.Id.Make.mkDataConRep` for the place where
   this check is implemented.
+
+* Although `type data` data constructors do not exist at the value level,
+  it is still possible to match on a value whose type is headed by a `type data`
+  type constructor, such as this example from #22964:
+
+    type data T a where
+      A :: T Int
+      B :: T a
+
+    f :: T a -> ()
+    f x = case x of {}
+
+  During checking the coverage of `f`'s pattern matches, we treat `T` as if it
+  were an empty data type so that GHC does not warn the user to match against
+  `A` or `B`.
 -}
 
 warnNoDerivStrat :: Maybe (LDerivStrategy GhcRn)


=====================================
testsuite/tests/pmcheck/should_compile/T22964.hs
=====================================
@@ -0,0 +1,15 @@
+{-# LANGUAGE TypeData #-}
+module X where
+
+type data T1 a where
+  A1 :: T1 Int
+  B1 :: T1 a
+
+f1 :: T1 a -> ()
+f1 x = case x of {}
+
+type data T2 a where
+  A2 :: T2 Int
+
+f2 :: T2 a -> ()
+f2 x = case x of {}


=====================================
testsuite/tests/pmcheck/should_compile/all.T
=====================================
@@ -158,3 +158,4 @@ test('EmptyCase009', [],  compile, [overlapping_incomplete])
 test('EmptyCase010', [],  compile, [overlapping_incomplete])
 test('T19271', [],  compile, [overlapping_incomplete])
 test('T21761', [],  compile, [overlapping_incomplete])
+test('T22964', [], compile, [overlapping_incomplete])



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/78dc538f31dd54f47f956e1ca66b75ce8251dc1c

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/78dc538f31dd54f47f956e1ca66b75ce8251dc1c
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