Add a recursivity check in nonVoid

Summary:
Previously `nonVoid` outright refused to call itself
recursively to avoid the risk of hitting infinite loops when
checking recurisve types. But this is too conservative—we //can//
call `nonVoid` recursively as long as we incorporate a way to detect
the presence of recursive types, and bail out if we do detect one.
Happily, such a mechanism already exists in the form of `checkRecTc`,
so let's use it.

Test Plan: make test TEST=T15584

Reviewers: simonpj, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, carter

GHC Trac Issues: #15584

Differential Revision: https://phabricator.haskell.org/D5116

2 files changed, 17 insertions, 0 deletions

diff --git a/testsuite/tests/pmcheck/should_compile/T15584.hs b/testsuite/tests/pmcheck/should_compile/T15584.hs
new file mode 100644
index 0000000000..c5d38bb32f
--- /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T15584.hs
@@ -0,0 +1,15 @@
+{-# LANGUAGE EmptyCase #-}
+{-# OPTIONS -Wincomplete-patterns #-}
+module T15584 where
+
+import Data.Void
+
+data V = MkV !Void
+data    S1 = MkS1 !V
+newtype S2 = MkS2 V
+
+s1 :: S1 -> a
+s1 x = case x of {}
+
+s2 :: S2 -> a
+s2 x = case x of {}
diff --git a/testsuite/tests/pmcheck/should_compile/all.T b/testsuite/tests/pmcheck/should_compile/all.T
index f2bbfffc00..20eef3ff95 100644
--- a/testsuite/tests/pmcheck/should_compile/all.T
+++ b/testsuite/tests/pmcheck/should_compile/all.T
@@ -69,6 +69,8 @@ test('T15385', normal, compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 test('T15450', normal, compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
+test('T15584', normal, compile,
+     ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 
 # Other tests
 test('pmc001', [], compile,