Simple subsumptionwip/T17775

This patch simplifies GHC to use simple subsumption.
  Ticket #17775

Implements GHC proposal #287
   https://github.com/ghc-proposals/ghc-proposals/blob/master/
   proposals/0287-simplify-subsumption.rst

All the motivation is described there; I will not repeat it here.
The implementation payload:
 * tcSubType and friends become noticably simpler, because it no
   longer uses eta-expansion when checking subsumption.
 * No deeplyInstantiate or deeplySkolemise

That in turn means that some tests fail, by design; they can all
be fixed by eta expansion.  There is a list of such changes below.

Implementing the patch led me into a variety of sticky corners, so
the patch includes several othe changes, some quite significant:

* I made String wired-in, so that
    "foo" :: String   rather than
    "foo" :: [Char]
  This improves error messages, and fixes #15679

* The pattern match checker relies on knowing about in-scope equality
  constraints, andd adds them to the desugarer's environment using
  addTyCsDs.  But the co_fn in a FunBind was missed, and for some reason
  simple-subsumption ends up with dictionaries there. So I added a
  call to addTyCsDs.  This is really part of #18049.

* I moved the ic_telescope field out of Implication and into
  ForAllSkol instead.  This is a nice win; just expresses the code
  much better.

* There was a bug in GHC.Tc.TyCl.Instance.tcDataFamInstHeader.
  We called checkDataKindSig inside tc_kind_sig, /before/
  solveEqualities and zonking.  Obviously wrong, easily fixed.

* solveLocalEqualitiesX: there was a whole mess in here, around
  failing fast enough.  I discovered a bad latent bug where we
  could successfully kind-check a type signature, and use it,
  but have unsolved constraints that could fill in coercion
  holes in that signature --  aargh.

  It's all explained in Note [Failure in local type signatures]
  in GHC.Tc.Solver. Much better now.

* I fixed a serious bug in anonymous type holes. IN
    f :: Int -> (forall a. a -> _) -> Int
  that "_" should be a unification variable at the /outer/
  level; it cannot be instantiated to 'a'.  This was plain
  wrong.  New fields mode_lvl and mode_holes in TcTyMode,
  and auxiliary data type GHC.Tc.Gen.HsType.HoleMode.

  This fixes #16292, but makes no progress towards the more
  ambitious #16082

* I got sucked into an enormous refactoring of the reporting of
  equality errors in GHC.Tc.Errors, especially in
      mkEqErr1
      mkTyVarEqErr
      misMatchMsg
      misMatchMsgOrCND
  In particular, the very tricky mkExpectedActualMsg function
  is gone.

  It took me a full day.  But the result is far easier to understand.
  (Still not easy!)  This led to various minor improvements in error
  output, and an enormous number of test-case error wibbles.

  One particular point: for occurs-check errors I now just say
     Can't match 'a' against '[a]'
  rather than using the intimidating language of "occurs check".

* Pretty-printing AbsBinds

Tests review

* Eta expansions
   T11305: one eta expansion
   T12082: one eta expansion (undefined)
   T13585a: one eta expansion
   T3102:  one eta expansion
   T3692:  two eta expansions (tricky)
   T2239:  two eta expansions
   T16473: one eta
   determ004: two eta expansions (undefined)
   annfail06: two eta (undefined)
   T17923: four eta expansions (a strange program indeed!)
   tcrun035: one eta expansion

* Ambiguity check at higher rank.  Now that we have simple
  subsumption, a type like
     f :: (forall a. Eq a => Int) -> Int
  is no longer ambiguous, because we could write
     g :: (forall a. Eq a => Int) -> Int
     g = f
  and it'd typecheck just fine.  But f's type is a bit
  suspicious, and we might want to consider making the
  ambiguity check do a check on each sub-term.  Meanwhile,
  these tests are accepted, whereas they were previously
  rejected as ambiguous:
     T7220a
     T15438
     T10503
     T9222

* Some more interesting error message wibbles
   T13381: Fine: one error (Int ~ Exp Int)
           rather than two (Int ~ Exp Int, Exp Int ~ Int)
   T9834:  Small change in error (improvement)
   T10619: Improved
   T2414:  Small change, due to order of unification, fine
   T2534:  A very simple case in which a change of unification order
           means we get tow unsolved constraints instead of one
   tc211: bizarre impredicative tests; just accept this for now

Updates Cabal and haddock submodules.

Metric Increase:
  T12150
  T12234
  T5837
  haddock.base
Metric Decrease:
  haddock.compiler
  haddock.Cabal
  haddock.base

Merge note: This appears to break the
`UnliftedNewtypesDifficultUnification` test. It has been marked as
broken in the interest of merging.

(cherry picked from commit 66b7b195cb3dce93ed5078b80bf568efae904cc5)

2 files changed, 23 insertions, 11 deletions

diff --git a/compiler/GHC/Builtin/Names.hs b/compiler/GHC/Builtin/Names.hs
index 9eb5061b64..8d7cecafdf 100644
--- a/compiler/GHC/Builtin/Names.hs
+++ b/compiler/GHC/Builtin/Names.hs
@@ -743,8 +743,7 @@ toInteger_RDR           = nameRdrName toIntegerName
 toRational_RDR          = nameRdrName toRationalName
 fromIntegral_RDR        = nameRdrName fromIntegralName
 
-stringTy_RDR, fromString_RDR :: RdrName
-stringTy_RDR            = tcQual_RDR gHC_BASE (fsLit "String")
+fromString_RDR :: RdrName
 fromString_RDR          = nameRdrName fromStringName
 
 fromList_RDR, fromListN_RDR, toList_RDR :: RdrName
@@ -1680,11 +1679,13 @@ addrPrimTyConKey, arrayPrimTyConKey, arrayArrayPrimTyConKey, boolTyConKey,
     mutableByteArrayPrimTyConKey, orderingTyConKey, mVarPrimTyConKey,
     ratioTyConKey, rationalTyConKey, realWorldTyConKey, stablePtrPrimTyConKey,
     stablePtrTyConKey, eqTyConKey, heqTyConKey,
-    smallArrayPrimTyConKey, smallMutableArrayPrimTyConKey :: Unique
+    smallArrayPrimTyConKey, smallMutableArrayPrimTyConKey,
+    stringTyConKey :: Unique
 addrPrimTyConKey                        = mkPreludeTyConUnique  1
 arrayPrimTyConKey                       = mkPreludeTyConUnique  3
 boolTyConKey                            = mkPreludeTyConUnique  4
 byteArrayPrimTyConKey                   = mkPreludeTyConUnique  5
+stringTyConKey                          = mkPreludeTyConUnique  6
 charPrimTyConKey                        = mkPreludeTyConUnique  7
 charTyConKey                            = mkPreludeTyConUnique  8
 doublePrimTyConKey                      = mkPreludeTyConUnique  9
diff --git a/compiler/GHC/Builtin/Types.hs b/compiler/GHC/Builtin/Types.hs
index d51f2243ca..eed9420aa6 100644
--- a/compiler/GHC/Builtin/Types.hs
+++ b/compiler/GHC/Builtin/Types.hs
@@ -39,7 +39,7 @@ module GHC.Builtin.Types (
 
         -- * Char
         charTyCon, charDataCon, charTyCon_RDR,
-        charTy, stringTy, charTyConName,
+        charTy, stringTy, charTyConName, stringTyCon_RDR,
 
         -- * Double
         doubleTyCon, doubleDataCon, doubleTy, doubleTyConName,
@@ -221,6 +221,7 @@ wiredInTyCons = [ -- Units are not treated like other tuples, because they
                 , anyTyCon
                 , boolTyCon
                 , charTyCon
+                , stringTyCon
                 , doubleTyCon
                 , floatTyCon
                 , intTyCon
@@ -301,11 +302,12 @@ coercibleTyConName   = mkWiredInTyConName   UserSyntax gHC_TYPES (fsLit "Coercib
 coercibleDataConName = mkWiredInDataConName UserSyntax gHC_TYPES (fsLit "MkCoercible") coercibleDataConKey coercibleDataCon
 coercibleSCSelIdName = mkWiredInIdName gHC_TYPES (fsLit "coercible_sel") coercibleSCSelIdKey coercibleSCSelId
 
-charTyConName, charDataConName, intTyConName, intDataConName :: Name
-charTyConName     = mkWiredInTyConName   UserSyntax gHC_TYPES (fsLit "Char") charTyConKey charTyCon
-charDataConName   = mkWiredInDataConName UserSyntax gHC_TYPES (fsLit "C#") charDataConKey charDataCon
-intTyConName      = mkWiredInTyConName   UserSyntax gHC_TYPES (fsLit "Int") intTyConKey   intTyCon
-intDataConName    = mkWiredInDataConName UserSyntax gHC_TYPES (fsLit "I#") intDataConKey  intDataCon
+charTyConName, charDataConName, intTyConName, intDataConName, stringTyConName :: Name
+charTyConName     = mkWiredInTyConName   UserSyntax gHC_TYPES (fsLit "Char")   charTyConKey charTyCon
+charDataConName   = mkWiredInDataConName UserSyntax gHC_TYPES (fsLit "C#")     charDataConKey charDataCon
+stringTyConName   = mkWiredInTyConName   UserSyntax gHC_BASE  (fsLit "String") stringTyConKey stringTyCon
+intTyConName      = mkWiredInTyConName   UserSyntax gHC_TYPES (fsLit "Int")    intTyConKey   intTyCon
+intDataConName    = mkWiredInDataConName UserSyntax gHC_TYPES (fsLit "I#")     intDataConKey  intDataCon
 
 boolTyConName, falseDataConName, trueDataConName :: Name
 boolTyConName     = mkWiredInTyConName   UserSyntax gHC_TYPES (fsLit "Bool") boolTyConKey boolTyCon
@@ -507,13 +509,14 @@ vecElemDataConNames = zipWith3Lazy mk_special_dc_name
 mk_special_dc_name :: FastString -> Unique -> DataCon -> Name
 mk_special_dc_name fs u dc = mkWiredInDataConName UserSyntax gHC_TYPES fs u dc
 
-boolTyCon_RDR, false_RDR, true_RDR, intTyCon_RDR, charTyCon_RDR,
+boolTyCon_RDR, false_RDR, true_RDR, intTyCon_RDR, charTyCon_RDR, stringTyCon_RDR,
     intDataCon_RDR, listTyCon_RDR, consDataCon_RDR :: RdrName
 boolTyCon_RDR   = nameRdrName boolTyConName
 false_RDR       = nameRdrName falseDataConName
 true_RDR        = nameRdrName trueDataConName
 intTyCon_RDR    = nameRdrName intTyConName
 charTyCon_RDR   = nameRdrName charTyConName
+stringTyCon_RDR = nameRdrName stringTyConName
 intDataCon_RDR  = nameRdrName intDataConName
 listTyCon_RDR   = nameRdrName listTyConName
 consDataCon_RDR = nameRdrName consDataConName
@@ -1402,7 +1405,15 @@ charDataCon :: DataCon
 charDataCon = pcDataCon charDataConName [] [charPrimTy] charTyCon
 
 stringTy :: Type
-stringTy = mkListTy charTy -- convenience only
+stringTy = mkTyConApp stringTyCon []
+
+stringTyCon :: TyCon
+-- We have this wired-in so that Haskell literal strings
+-- get type String (in hsLitType), which in turn influences
+-- inferred types and error messages
+stringTyCon = buildSynTyCon stringTyConName
+                            [] liftedTypeKind []
+                            (mkListTy charTy)
 
 intTy :: Type
 intTy = mkTyConTy intTyCon