PmCheck: Print types of uncovered patterns (#18932)wip/T18932

In order to avoid confusion as in #18932, we display the type of the
match variables in the non-exhaustiveness warning, e.g.

```
T18932.hs:14:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘g’:
        Patterns of type  ‘T a’, ‘T a’, ‘T a’ not matched:
            (MkT2 _) (MkT1 _) (MkT1 _)
            (MkT2 _) (MkT1 _) (MkT2 _)
            (MkT2 _) (MkT2 _) (MkT1 _)
            (MkT2 _) (MkT2 _) (MkT2 _)
            ...
   |
14 | g (MkT1 x) (MkT1 _) (MkT1 _) = x
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

It also allows us to omit the type signature on wildcard matches which
we previously showed in only some situations, particularly
`-XEmptyCase`.

Fixes #18932.

2 files changed, 6 insertions, 16 deletions

diff --git a/compiler/GHC/HsToCore/Pmc.hs b/compiler/GHC/HsToCore/Pmc.hs
index 7af0d4605e..651f37f909 100644
--- a/compiler/GHC/HsToCore/Pmc.hs
+++ b/compiler/GHC/HsToCore/Pmc.hs
@@ -381,7 +381,10 @@ reportWarnings dflags ctx@(DsMatchContext kind loc) vars
       case vars of -- See #11245
            [] -> text "Guards do not cover entire pattern space"
            _  -> let us = map (\nabla -> pprUncovered nabla vars) nablas
-                 in  hang (text "Patterns not matched:") 4
+                     pp_tys = pprQuotedList $ map idType vars
+                 in  hang
+                       (text "Patterns of type" <+> pp_tys <+> text "not matched:")
+                       4
                        (vcat (take maxPatterns us) $$ dots maxPatterns us)
 
     approx_msg = vcat
diff --git a/compiler/GHC/HsToCore/Pmc/Ppr.hs b/compiler/GHC/HsToCore/Pmc/Ppr.hs
index fea1ecfe39..3de6a14970 100644
--- a/compiler/GHC/HsToCore/Pmc/Ppr.hs
+++ b/compiler/GHC/HsToCore/Pmc/Ppr.hs
@@ -136,26 +136,13 @@ checkRefuts x = do
 
 -- | Pretty print a variable, but remember to prettify the names of the variables
 -- that refer to neg-literals. The ones that cannot be shown are printed as
--- underscores. Even with a type signature, if it's not too noisy.
+-- underscores.
 pprPmVar :: PprPrec -> Id -> PmPprM SDoc
--- Type signature is "too noisy" by my definition if it needs to parenthesize.
--- I like           "not matched: _ :: Proxy (DIdEnv (Id, SDoc))",
--- but I don't like "not matched: (_ :: stuff) (_:_) (_ :: Proxy (DIdEnv (Id, SDoc)))"
--- The useful information in the latter case is the constructor that we missed,
--- not the types of the wildcards in the places that aren't matched as a result.
 pprPmVar prec x = do
   nabla <- ask
   case lookupSolution nabla x of
     Just (PACA alt _tvs args) -> pprPmAltCon prec alt args
-    Nothing                   -> fromMaybe typed_wildcard <$> checkRefuts x
-      where
-        -- if we have no info about the parameter and would just print a
-        -- wildcard, also show its type.
-        typed_wildcard
-          | prec <= sigPrec
-          = underscore <+> text "::" <+> ppr (idType x)
-          | otherwise
-          = underscore
+    Nothing                   -> fromMaybe underscore <$> checkRefuts x
 
 pprPmAltCon :: PprPrec -> PmAltCon -> [Id] -> PmPprM SDoc
 pprPmAltCon _prec (PmAltLit l)      _    = pure (ppr l)