Distinguish Inferred from Specified tyvars

In a declared type we need to distinguish between Inferred
and Specified type variables. This was exposed by Trac #15592.

See Note [Work out final tyConBinders] in TcTyClsDecls.

I had to change the definition of HasField in GHC.Records to
   class HasField x r a | x r -> a where
so as to have an /inferred/ kind argument rather than a
specfied one.  So
   HasField :: forall {k}. k -> * -> * -> Constraint

1 files changed, 8 insertions, 1 deletions

diff --git a/libraries/base/GHC/Records.hs b/libraries/base/GHC/Records.hs
index 43c3931e86..3b1a4c2834 100644
--- a/libraries/base/GHC/Records.hs
+++ b/libraries/base/GHC/Records.hs
@@ -29,6 +29,13 @@ module GHC.Records
 -- | Constraint representing the fact that the field @x@ belongs to
 -- the record type @r@ and has field type @a@.  This will be solved
 -- automatically, but manual instances may be provided as well.
-class HasField (x :: k) r a | x r -> a where
+--
+--   HasField :: forall {k}. k -> * -> * -> Constraint
+--   getField :: forall {k} (x::k) r a. HasField x r a => r -> a
+-- NB: The {k} means that k is an 'inferred' type variable, and
+--     hence not provided in visible type applications.  Thus you
+--     say     getField @"foo"
+--     not     getField @Symbol @"foo"
+class HasField x r a | x r -> a where
   -- | Selector function to extract the field from the record.
   getField :: r -> a