Look inside implications in simplifyRule

Trac #14732 was a perpelexing bug in which -fdefer-typed-holes
caused a mysterious type error in a RULE.  This turned out to
be because we are more aggressive about creating implications
when deferring (see TcUnify.implicationNeeded), and the rule
mechanism hadn't caught up.

This fixes it.

2 files changed, 35 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T14732.hs b/testsuite/tests/typecheck/should_compile/T14732.hs
new file mode 100644
index 0000000000..4fa070ed09
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T14732.hs
@@ -0,0 +1,34 @@
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# OPTIONS_GHC -fdefer-type-errors -O #-}
+-- Oddly this bug was only triggered with -fdefer-type-errors
+-- The -O ensures that the RULE is processed
+
+module T14732 where
+
+import Prelude hiding (zip, zipWith)
+
+zipWith :: (a -> b -> c)
+        -> Bundle v a
+        -> Bundle v b
+        -> Bundle v c
+zipWith = undefined
+
+class GVector (v :: * -> *) a
+instance GVector Vector a
+
+data Bundle (v :: * -> *) a
+data Vector a
+class Unbox a
+
+stream :: GVector v a => v a -> Bundle v a
+{-# INLINE [1] stream #-}
+stream = undefined
+
+zip :: (Unbox a, Unbox b) => Vector a -> Vector b -> Vector (a, b)
+{-# INLINE [1] zip #-}
+zip = undefined
+{-# RULES "stream/zip [Vector.Unboxed]" forall as bs .
+  stream (zip as bs) = zipWith (,) (stream as)
+                                   (stream bs)   #-}
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 9e6898190d..795e1730a9 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -591,3 +591,4 @@ test('T14488', normal, compile, [''])
 test('T14590', normal, compile, ['-fdefer-type-errors -fno-max-valid-substitutions'])
 test('T13032', normal, compile, [''])
 test('T14273', normal, compile, ['-fdefer-type-errors -fno-max-valid-substitutions'])
+test('T14732', normal, compile, [''])