Add AnonArgFlag to FunTy

The big payload of this patch is:

  Add an AnonArgFlag to the FunTy constructor
  of Type, so that
    (FunTy VisArg   t1 t2) means (t1 -> t2)
    (FunTy InvisArg t1 t2) means (t1 => t2)

The big payoff is that we have a simple, local test to make
when decomposing a type, leading to many fewer calls to
isPredTy. To me the code seems a lot tidier, and probably
more efficient (isPredTy has to take the kind of the type).

See Note [Function types] in TyCoRep.

There are lots of consequences

* I made FunTy into a record, so that it'll be easier
  when we add a linearity field, something that is coming
  down the road.

* Lots of code gets touched in a routine way, simply because it
  pattern matches on FunTy.

* I wanted to make a pattern synonym for (FunTy2 arg res), which
  picks out just the argument and result type from the record. But
  alas the pattern-match overlap checker has a heart attack, and
  either reports false positives, or takes too long.  In the end
  I gave up on pattern synonyms.

  There's some commented-out code in TyCoRep that shows what I
  wanted to do.

* Much more clarity about predicate types, constraint types
  and (in particular) equality constraints in kinds.  See TyCoRep
  Note [Types for coercions, predicates, and evidence]
  and Note [Constraints in kinds].

  This made me realise that we need an AnonArgFlag on
  AnonTCB in a TyConBinder, something that was really plain
  wrong before. See TyCon Note [AnonTCB InivsArg]

* When building function types we must know whether we
  need VisArg (mkVisFunTy) or InvisArg (mkInvisFunTy).
  This turned out to be pretty easy in practice.

* Pretty-printing of types, esp in IfaceType, gets
  tidier, because we were already recording the (->)
  vs (=>) distinction in an ad-hoc way.  Death to
  IfaceFunTy.

* mkLamType needs to keep track of whether it is building
  (t1 -> t2) or (t1 => t2).  See Type
  Note [mkLamType: dictionary arguments]

Other minor stuff

* Some tidy-up in validity checking involving constraints;
  Trac #16263

2 files changed, 10 insertions, 5 deletions

diff --git a/testsuite/tests/dependent/should_fail/T15215.hs b/testsuite/tests/dependent/should_fail/T15215.hs
index 96fe04385b..e4968a1654 100644
--- a/testsuite/tests/dependent/should_fail/T15215.hs
+++ b/testsuite/tests/dependent/should_fail/T15215.hs
@@ -8,5 +8,9 @@ import Data.Kind
 data A :: Type -> Type where
   MkA :: Show (Maybe a) => A a
 
+data B :: Type -> Type where
+  MkB :: Show a => B a
+
 data SA :: forall a. A a -> Type where
   SMkA :: SA MkA
+  SMkB :: SA MkB
diff --git a/testsuite/tests/dependent/should_fail/T15215.stderr b/testsuite/tests/dependent/should_fail/T15215.stderr
index 53aff765a3..be2d7c75e7 100644
--- a/testsuite/tests/dependent/should_fail/T15215.stderr
+++ b/testsuite/tests/dependent/should_fail/T15215.stderr
@@ -5,8 +5,9 @@ T15215.hs:9:3: error:
     • In the definition of data constructor ‘MkA’
       In the data type declaration for ‘A’
 
-T15215.hs:12:14: error:
-    • Illegal constraint in a kind: Show (Maybe a0)
-    • In the first argument of ‘SA’, namely ‘MkA’
-      In the type ‘SA MkA’
-      In the definition of data constructor ‘SMkA’
+T15215.hs:16:14: error:
+    • Data constructor ‘MkB’ cannot be used here
+        (it has an unpromotable context ‘Show a’)
+    • In the first argument of ‘SA’, namely ‘MkB’
+      In the type ‘SA MkB’
+      In the definition of data constructor ‘SMkB’