Push coercions in exprIsConApp_maybe

Trac #13025 showed up the fact that exprIsConApp_maybe isn't
clever enough: it didn't push coercions through applicatins, and that
meant we weren't getting as much superclass selection as we should.

It's easy to fix, happily.

See Note [Push coercions in exprIsConApp_maybe]

1 files changed, 15 insertions, 0 deletions

diff --git a/testsuite/tests/simplCore/should_compile/T13025.hs b/testsuite/tests/simplCore/should_compile/T13025.hs
new file mode 100644
index 0000000000..01facb8601
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T13025.hs
@@ -0,0 +1,15 @@
+{-# LANGUAGE DataKinds #-}
+module T13025 where
+import T13025a
+
+type MyRec = Rec '[ '("A",Int), '("B",Int), '("C",Int) ]
+
+getC :: MyRec -> Int
+getC = getField (Proxy::Proxy '("C",Int))
+
+doubleC :: MyRec -> MyRec
+doubleC r = setC (2 * (getC r)) r
+  where setC = set . (Field :: Int -> Field '("C",Int))
+
+main :: IO ()
+main = print (getC (Field 1 :& Field 2 :& Field 3 :& Nil :: MyRec))