Improve typechecking of instance defaults

In an instance declaration when you don't specify the code for a
method, GHC fills in from the default binding in the class.
The type of the default method can legitmiately be ambiguous ---
see Note [Default methods in instances] in TcInstDcls --- so
typechecking it can be tricky.

Trac #12220 showed that although we were dealing with that ambiguity
for /vanilla/ default methods, we were not doing so for /generic/
default methods.  Moreover we were dealing with it clumsily, by
generating post-typechecked code.

This patch fixes the bug AND deletes code!  We now use the same code
path for both vanilla and generic default methods; and generate
/pre-typechecked/ code in both cases.  The key trick is that we can use
Visible Type Application to deal with the ambiguity, which wasn't
possible before.  Hooray.

There is a small hit to performance in compiler/perf/T1969 which
consists of nothing BUT instance declarations with several default
methods to fill, which we now have to typecheck. The actual hit is
from 724 -> 756 or 4% in that extreme example.  Real world programs
have vastly fewer instance decls.

1 files changed, 37 insertions, 0 deletions

diff --git a/testsuite/tests/generics/T12220.hs b/testsuite/tests/generics/T12220.hs
new file mode 100755
index 0000000000..70f8b58f84
--- /dev/null
+++ b/testsuite/tests/generics/T12220.hs
@@ -0,0 +1,37 @@
+{-#LANGUAGE TypeApplications#-}
+{-#LANGUAGE MultiParamTypeClasses #-}
+{-#LANGUAGE AllowAmbiguousTypes #-}
+{-#LANGUAGE FlexibleInstances #-}
+{-#LANGUAGE ScopedTypeVariables #-}
+{-#LANGUAGE DefaultSignatures #-}
+module T12220 where
+
+-- | Type a is only used for
+-- type application.
+class ToUse a where
+    toUse :: Int -> Int
+
+-- | The type used for
+-- type application
+data Default
+
+
+-- | The instance using Default as type application.
+-- To call use:
+-- > toUse @Default
+instance ToUse Default where
+    toUse a = 3*a
+
+-- | Typeclass whose methods work
+-- only with type application.
+class Uses a b where
+    uses :: b -> [b]
+    -- | Default Signature, which generates the problem.
+    -- It is the same as the normal one
+    -- Comment it to 'fix' the bug.
+    default uses :: b -> [b]
+    uses v = [v]
+
+-- | But this one doesn't.
+-- Unless you comment the default signature.
+instance (Uses t a, Uses t b, Uses t c) => Uses t (a,b,c)