Exhaustiveness check for EmptyCase (Trac #10746)

Empty case expressions have strict semantics so the problem boils down
to inhabitation checking for the type of the scrutinee. 3 main functions
added:

- pmTopNormaliseType_maybe for the normalisation of the scrutinee type

- inhabitationCandidates for generating the possible patterns of the
  appropriate type

- checkEmptyCase' to filter out the candidates that give rise to
  unsatisfiable constraints.

See Note [Checking EmptyCase Expressions] in Check
and Note [Type normalisation for EmptyCase] in FamInstEnv

Test Plan: validate

Reviewers: simonpj, goldfire, dfeuer, austin, bgamari

Reviewed By: bgamari

Subscribers: mpickering, thomie

Differential Revision: https://phabricator.haskell.org/D2105

GHC Trac Issues: #10746

1 files changed, 41 insertions, 0 deletions

diff --git a/testsuite/tests/pmcheck/should_compile/EmptyCase010.stderr b/testsuite/tests/pmcheck/should_compile/EmptyCase010.stderr
new file mode 100644
index 0000000000..d4ccce34bb
--- /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/EmptyCase010.stderr
@@ -0,0 +1,41 @@
+EmptyCase010.hs:22:7: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In a case alternative:
+        Patterns not matched:
+            (Baz MkGC1)
+            (Baz (MkGC2 _))
+
+EmptyCase010.hs:26:7: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In a case alternative: Patterns not matched: (Baz MkGC1)
+
+EmptyCase010.hs:35:7: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In a case alternative:
+        Patterns not matched:
+            (Baz MkGD1)
+            (Baz MkGD3)
+
+EmptyCase010.hs:39:9: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In a case alternative: Patterns not matched: (Baz MkGD3)
+
+EmptyCase010.hs:43:7: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In a case alternative:
+        Patterns not matched:
+            (Baz MkGD1)
+            (Baz MkGD2)
+            (Baz MkGD3)
+
+EmptyCase010.hs:55:7: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In a case alternative: Patterns not matched: _ :: Baz (DC ()) a
+
+EmptyCase010.hs:67:7: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In a case alternative: Patterns not matched: (Baz MkDC2)
+
+EmptyCase010.hs:71:9: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In a case alternative: Patterns not matched: _ :: Baz f a