Implement Quick Look impredicativity

This patch implements Quick Look impredicativity (#18126), sticking
very closely to the design in
    A quick look at impredicativity, Serrano et al, ICFP 2020

The main change is that a big chunk of GHC.Tc.Gen.Expr has been
extracted to two new modules
    GHC.Tc.Gen.App
    GHC.Tc.Gen.Head
which deal with typechecking n-ary applications, and the head of
such applications, respectively.  Both contain a good deal of
documentation.

Three other loosely-related changes are in this patch:

* I implemented (partly by accident) points (2,3)) of the accepted GHC
  proposal "Clean up printing of foralls", namely
  https://github.com/ghc-proposals/ghc-proposals/blob/
        master/proposals/0179-printing-foralls.rst
  (see #16320).

  In particular, see Note [TcRnExprMode] in GHC.Tc.Module
  - :type instantiates /inferred/, but not /specified/, quantifiers
  - :type +d instantiates /all/ quantifiers
  - :type +v is killed off

  That completes the implementation of the proposal,
  since point (1) was done in
    commit df08468113ab46832b7ac0a7311b608d1b418c4d
    Author: Krzysztof Gogolewski <krzysztof.gogolewski@tweag.io>
    Date:   Mon Feb 3 21:17:11 2020 +0100
    Always display inferred variables using braces

* HsRecFld (which the renamer introduces for record field selectors),
  is now preserved by the typechecker, rather than being rewritten
  back to HsVar.  This is more uniform, and turned out to be more
  convenient in the new scheme of things.

* The GHCi debugger uses a non-standard unification that allows the
  unification variables to unify with polytypes.  We used to hack
  this by using ImpredicativeTypes, but that doesn't work anymore
  so I introduces RuntimeUnkTv.  See Note [RuntimeUnkTv] in
  GHC.Runtime.Heap.Inspect

Updates haddock submodule.

WARNING: this patch won't validate on its own.  It was too
hard to fully disentangle it from the following patch, on
type errors and kind generalisation.

Changes to tests

* Fixes #9730 (test added)

* Fixes #7026 (test added)

* Fixes most of #8808, except function `g2'` which uses a
  section (which doesn't play with QL yet -- see #18126)
  Test added

* Fixes #1330. NB Church1.hs subsumes Church2.hs, which is now deleted

* Fixes #17332 (test added)

* Fixes #4295

* This patch makes typecheck/should_run/T7861 fail.
  But that turns out to be a pre-existing bug: #18467.
  So I have just made T7861 into expect_broken(18467)

1 files changed, 19 insertions, 14 deletions

diff --git a/compiler/GHC/Hs/Expr.hs b/compiler/GHC/Hs/Expr.hs
index 251fae2c48..11a102f91b 100644
--- a/compiler/GHC/Hs/Expr.hs
+++ b/compiler/GHC/Hs/Expr.hs
@@ -40,6 +40,7 @@ import GHC.Hs.Binds
 -- others:
 import GHC.Tc.Types.Evidence
 import GHC.Core
+import GHC.Types.Id( Id )
 import GHC.Types.Name
 import GHC.Types.Name.Set
 import GHC.Types.Basic
@@ -251,8 +252,8 @@ data HsExpr p
                              -- Turned from HsVar to HsUnboundVar by the
                              --   renamer, when it finds an out-of-scope
                              --   variable or hole.
-                             -- Turned into HsVar by type checker, to support
-                             --   deferred type errors.
+                             -- The (XUnboundVar p) field becomes Id
+                             --   after typechecking
 
   | HsConLikeOut (XConLikeOut p)
                  ConLike     -- ^ After typechecker only; must be different
@@ -260,7 +261,9 @@ data HsExpr p
 
   | HsRecFld  (XRecFld p)
               (AmbiguousFieldOcc p) -- ^ Variable pointing to record selector
-                                    -- Not in use after typechecking
+              -- The parser produces HsVars
+              -- The renamer renames record-field selectors to HsRecFld
+              -- The typechecker preserves HsRecFld
 
   | HsOverLabel (XOverLabel p)
                 (Maybe (IdP p)) FastString
@@ -592,7 +595,6 @@ deriving instance (Data (hs_syn GhcTc), Typeable hs_syn) => Data (HsWrap hs_syn)
 -- ---------------------------------------------------------------------
 
 type instance XVar           (GhcPass _) = NoExtField
-type instance XUnboundVar    (GhcPass _) = NoExtField
 type instance XConLikeOut    (GhcPass _) = NoExtField
 type instance XRecFld        (GhcPass _) = NoExtField
 type instance XOverLabel     (GhcPass _) = NoExtField
@@ -603,6 +605,10 @@ type instance XLam           (GhcPass _) = NoExtField
 type instance XLamCase       (GhcPass _) = NoExtField
 type instance XApp           (GhcPass _) = NoExtField
 
+type instance XUnboundVar    GhcPs = NoExtField
+type instance XUnboundVar    GhcRn = NoExtField
+type instance XUnboundVar    GhcTc = Id
+
 type instance XAppTypeE      GhcPs = NoExtField
 type instance XAppTypeE      GhcRn = NoExtField
 type instance XAppTypeE      GhcTc = Type
@@ -1049,14 +1055,15 @@ ppr_lexpr e = ppr_expr (unLoc e)
 
 ppr_expr :: forall p. (OutputableBndrId p)
          => HsExpr (GhcPass p) -> SDoc
-ppr_expr (HsVar _ (L _ v))  = pprPrefixOcc v
-ppr_expr (HsUnboundVar _ uv)= pprPrefixOcc uv
-ppr_expr (HsConLikeOut _ c) = pprPrefixOcc c
-ppr_expr (HsIPVar _ v)      = ppr v
-ppr_expr (HsOverLabel _ _ l)= char '#' <> ppr l
-ppr_expr (HsLit _ lit)      = ppr lit
-ppr_expr (HsOverLit _ lit)  = ppr lit
-ppr_expr (HsPar _ e)        = parens (ppr_lexpr e)
+ppr_expr (HsVar _ (L _ v))   = pprPrefixOcc v
+ppr_expr (HsUnboundVar _ uv) = pprPrefixOcc uv
+ppr_expr (HsConLikeOut _ c)  = pprPrefixOcc c
+ppr_expr (HsRecFld _ f)      = pprPrefixOcc f
+ppr_expr (HsIPVar _ v)       = ppr v
+ppr_expr (HsOverLabel _ _ l) = char '#' <> ppr l
+ppr_expr (HsLit _ lit)       = ppr lit
+ppr_expr (HsOverLit _ lit)   = ppr lit
+ppr_expr (HsPar _ e)         = parens (ppr_lexpr e)
 
 ppr_expr (HsPragE _ prag e) = sep [ppr prag, ppr_lexpr e]
 
@@ -1212,7 +1219,6 @@ ppr_expr (HsBinTick _ tickIdTrue tickIdFalse exp)
           text ">(",
           ppr exp, text ")"]
 
-ppr_expr (HsRecFld _ f) = ppr f
 ppr_expr (XExpr x) = case ghcPass @p of
 #if __GLASGOW_HASKELL__ < 811
   GhcPs -> ppr x
@@ -1377,7 +1383,6 @@ isAtomicHsExpr (HsOverLit {})    = True
 isAtomicHsExpr (HsIPVar {})      = True
 isAtomicHsExpr (HsOverLabel {})  = True
 isAtomicHsExpr (HsUnboundVar {}) = True
-isAtomicHsExpr (HsPar _ e)       = isAtomicHsExpr (unLoc e)
 isAtomicHsExpr (HsRecFld{})      = True
 isAtomicHsExpr (XExpr x)
   | GhcTc <- ghcPass @p          = case x of