Fix map/coerce rule for newtypes with wrappers

This addresses Trac #16208 by marking newtype wrapper
unfoldings as compulsory.

Furthermore, we can remove the special case for newtypes
in exprIsConApp_maybe (introduced in 7833cf407d1f).

3 files changed, 19 insertions, 0 deletions

diff --git a/testsuite/tests/simplCore/should_run/T16208.hs b/testsuite/tests/simplCore/should_run/T16208.hs
new file mode 100644
index 0000000000..e346ab84f6
--- /dev/null
+++ b/testsuite/tests/simplCore/should_run/T16208.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE GADTs, ExplicitForAll #-}
+module Main (main) where
+
+import GHC.Exts
+
+newtype Age a b where
+  Age :: forall b a. Int -> Age a b
+
+data T a = MkT a
+
+{-# NOINLINE foo #-}
+foo :: (Int -> Age Bool Char) -> String
+foo _ = "bad (RULE should have fired)"
+
+{-# RULES "foo/coerce" [1] foo coerce = "good" #-}
+
+main = putStrLn (foo Age)
diff --git a/testsuite/tests/simplCore/should_run/T16208.stdout b/testsuite/tests/simplCore/should_run/T16208.stdout
new file mode 100644
index 0000000000..12799ccbe7
--- /dev/null
+++ b/testsuite/tests/simplCore/should_run/T16208.stdout
@@ -0,0 +1 @@
+good
diff --git a/testsuite/tests/simplCore/should_run/all.T b/testsuite/tests/simplCore/should_run/all.T
index f8089438c5..646929f778 100644
--- a/testsuite/tests/simplCore/should_run/all.T
+++ b/testsuite/tests/simplCore/should_run/all.T
@@ -50,6 +50,7 @@ test('T5441', [], multimod_compile_and_run, ['T5441', ''])
 test('T5603', reqlib('integer-gmp'), compile_and_run, [''])
 test('T2110', normal, compile_and_run, [''])
 test('AmapCoerce', normal, compile_and_run, [''])
+test('T16208', normal, compile_and_run, [''])
 
 # Run these tests *without* optimisation too
 test('T5625', [ only_ways(['normal','optasm']), exit_code(1) ], compile_and_run, [''])