Refactor ConDecl

The ConDecl type in HsDecls is an uneasy compromise. For the most part,
HsSyn directly reflects the syntax written by the programmer; and that
gives just the right "pegs" on which to hang Alan's API annotations. But
ConDecl doesn't properly reflect the syntax of Haskell-98 and GADT-style
data type declarations.

To be concrete, here's a draft new data type

```lang=hs
data ConDecl name
  | ConDeclGADT
      { con_names   :: [Located name]
      , con_type    :: LHsSigType name  -- The type after the ‘::’
      , con_doc     :: Maybe LHsDocString }

  | ConDeclH98
      { con_name    :: Located name

      , con_qvars     :: Maybe (LHsQTyVars name)
        -- User-written forall (if any), and its implicit
        -- kind variables
        -- Non-Nothing needs -XExistentialQuantification

      , con_cxt       :: Maybe (LHsContext name)
        -- ^ User-written context (if any)

      , con_details   :: HsConDeclDetails name
          -- ^ Arguments

      , con_doc       :: Maybe LHsDocString
          -- ^ A possible Haddock comment.
      } deriving (Typeable)
```

Note that

    For GADTs, just keep a type. That's what the user writes.
    NB:HsType can represent records on the LHS of an arrow:

      { x:Int,y:Bool} -> T

    con_qvars and con_cxt are both Maybe because they are both
    optional (the forall and the context of an existential data type

    For ConDeclGADT the type variables of the data type do not scope
    over the con_type; whereas for ConDeclH98 they do scope over con_cxt
    and con_details.

Updates haddock submodule.

Test Plan: ./validate

Reviewers: simonpj, erikd, hvr, goldfire, austin, bgamari

Subscribers: erikd, goldfire, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D1558

GHC Trac Issues: #11028

2 files changed, 6 insertions, 2 deletions

diff --git a/testsuite/tests/rename/should_compile/T5331.stderr b/testsuite/tests/rename/should_compile/T5331.stderr
index 13249b0e17..965e15a9b4 100644
--- a/testsuite/tests/rename/should_compile/T5331.stderr
+++ b/testsuite/tests/rename/should_compile/T5331.stderr
@@ -5,7 +5,7 @@ T5331.hs:8:17: warning:
 

 T5331.hs:11:16: warning:

     Unused quantified type variable ‘a’

-    In the definition of data constructor ‘W1’

+    In the type ‘forall a. W’

 

 T5331.hs:13:13: warning:

     Unused quantified type variable ‘a’

diff --git a/testsuite/tests/rename/should_fail/T7943.stderr b/testsuite/tests/rename/should_fail/T7943.stderr
index 8594a25e2f..c6bf7ae9b5 100644
--- a/testsuite/tests/rename/should_fail/T7943.stderr
+++ b/testsuite/tests/rename/should_fail/T7943.stderr
@@ -1,2 +1,6 @@
 
-T7943.hs:4:22: Record syntax is illegal here: {bar :: String}
+T7943.hs:4:22:
+     Record syntax is illegal here: {bar :: String}
+     In the type ‘{bar :: String}’
+      In the definition of data constructor ‘B’
+      In the data declaration for ‘Foo’
\ No newline at end of file