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)

1 files changed, 25 insertions, 23 deletions

diff --git a/compiler/GHC/Tc/Types/Constraint.hs b/compiler/GHC/Tc/Types/Constraint.hs
index 908a23ff26..3f01a7d03a 100644
--- a/compiler/GHC/Tc/Types/Constraint.hs
+++ b/compiler/GHC/Tc/Types/Constraint.hs
@@ -29,7 +29,7 @@ module GHC.Tc.Types.Constraint (
 
         WantedConstraints(..), insolubleWC, emptyWC, isEmptyWC,
         isSolvedWC, andWC, unionsWC, mkSimpleWC, mkImplicWC,
-        addInsols, insolublesOnly, addSimples, addImplics, addHole,
+        addInsols, dropMisleading, addSimples, addImplics, addHoles,
         tyCoVarsOfWC, dropDerivedWC, dropDerivedSimples,
         tyCoVarsOfWCList, insolubleCt, insolubleEqCt,
         isDroppableCt, insolubleImplic,
@@ -961,19 +961,24 @@ addInsols :: WantedConstraints -> Bag Ct -> WantedConstraints
 addInsols wc cts
   = wc { wc_simple = wc_simple wc `unionBags` cts }
 
-addHole :: WantedConstraints -> Hole -> WantedConstraints
-addHole wc hole
-  = wc { wc_holes = hole `consBag` wc_holes wc }
+addHoles :: WantedConstraints -> Bag Hole -> WantedConstraints
+addHoles wc holes
+  = wc { wc_holes = holes `unionBags` wc_holes wc }
 
-insolublesOnly :: WantedConstraints -> WantedConstraints
--- Keep only the definitely-insoluble constraints
-insolublesOnly (WC { wc_simple = simples, wc_impl = implics, wc_holes = holes })
-  = WC { wc_simple = filterBag insolubleCt simples
-       , wc_impl   = mapBag implic_insols_only implics
+dropMisleading :: WantedConstraints -> WantedConstraints
+-- Drop misleading constraints; really just class constraints
+-- See Note [Constraints and errors] in GHC.Tc.Utils.Monad
+dropMisleading (WC { wc_simple = simples, wc_impl = implics, wc_holes = holes })
+  = WC { wc_simple = filterBag keep_ct simples
+       , wc_impl   = mapBag drop_implic implics
        , wc_holes  = filterBag isOutOfScopeHole holes }
   where
-    implic_insols_only implic
-      = implic { ic_wanted = insolublesOnly (ic_wanted implic) }
+    drop_implic implic
+      = implic { ic_wanted = dropMisleading (ic_wanted implic) }
+    keep_ct ct
+      = case classifyPredType (ctPred ct) of
+          ClassPred {} -> False
+          _ -> True
 
 isSolvedStatus :: ImplicStatus -> Bool
 isSolvedStatus (IC_Solved {}) = True
@@ -1100,9 +1105,6 @@ data Implication
       ic_info  :: SkolemInfo,    -- See Note [Skolems in an implication]
                                  -- See Note [Shadowing in a constraint]
 
-      ic_telescope :: Maybe SDoc,  -- User-written telescope, if there is one
-                                   -- See Note [Checking telescopes]
-
       ic_given  :: [EvVar],      -- Given evidence variables
                                  --   (order does not matter)
                                  -- See Invariant (GivenInv) in GHC.Tc.Utils.TcType
@@ -1153,7 +1155,6 @@ implicationPrototype
 
               -- The rest have sensible default values
             , ic_skols      = []
-            , ic_telescope  = Nothing
             , ic_given      = []
             , ic_wanted     = emptyWC
             , ic_no_eqs     = False
@@ -1228,17 +1229,18 @@ all at once, creating one implication constraint for the lot:
   variables (ic_skols).  This is done in setImplicationStatus.
 
 * This check is only necessary if the implication was born from a
-  user-written signature.  If, say, it comes from checking a pattern
-  match that binds existentials, where the type of the data constructor
-  is known to be valid (it in tcConPat), no need for the check.
+  'forall' in a user-written signature (the HsForAllTy case in
+  GHC.Tc.Gen.HsType.  If, say, it comes from checking a pattern match
+  that binds existentials, where the type of the data constructor is
+  known to be valid (it in tcConPat), no need for the check.
 
-  So the check is done if and only if ic_telescope is (Just blah).
+  So the check is done if and only if ic_info is ForAllSkol
 
-* If ic_telesope is (Just d), the d::SDoc displays the original,
-  user-written type variables.
+* If ic_info is (ForAllSkol dt dvs), the dvs::SDoc displays the
+  original, user-written type variables.
 
-* Be careful /NOT/ to discard an implication with non-Nothing
-  ic_telescope, even if ic_wanted is empty.  We must give the
+* Be careful /NOT/ to discard an implication with a ForAllSkol
+  ic_info, even if ic_wanted is empty.  We must give the
   constraint solver a chance to make that bad-telescope test!  Hence
   the extra guard in emitResidualTvConstraint; see #16247