Add right-to-left rule for pattern bindings

Fix #18323 by adding a few lines of code to handle non-recursive
pattern bindings.  see GHC.Tc.Gen.Bind
Note [Special case for non-recursive pattern bindings]

Alas, this confused the pattern-match overlap checker; see #18323.

Note that this patch only affects pattern bindings like that
for (x,y) in this program

  combine :: (forall a . [a] -> a) -> [forall a. a -> a]
          -> ((forall a . [a] -> a), [forall a. a -> a])

  breaks = let (x,y) = combine head ids
           in x y True

We need ImpredicativeTypes for those [forall a. a->a] types to be
valid. And with ImpredicativeTypes the old, unprincipled "allow
unification variables to unify with a polytype" story actually
works quite well. So this test compiles fine (if delicatedly) with
old GHCs; but not with QuickLook unless we add this patch

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/GHC/Tc/Gen/Match.hs-boot b/compiler/GHC/Tc/Gen/Match.hs-boot
index 692d04b884..bb194a3cf1 100644
--- a/compiler/GHC/Tc/Gen/Match.hs-boot
+++ b/compiler/GHC/Tc/Gen/Match.hs-boot
@@ -2,13 +2,13 @@ module GHC.Tc.Gen.Match where
 import GHC.Hs           ( GRHSs, MatchGroup, LHsExpr )
 import GHC.Tc.Types.Evidence  ( HsWrapper )
 import GHC.Types.Name   ( Name )
-import GHC.Tc.Utils.TcType( ExpSigmaType, TcRhoType )
+import GHC.Tc.Utils.TcType( ExpSigmaType, ExpRhoType )
 import GHC.Tc.Types     ( TcM )
 import GHC.Types.SrcLoc ( Located )
 import GHC.Hs.Extension ( GhcRn, GhcTc )
 
 tcGRHSsPat    :: GRHSs GhcRn (LHsExpr GhcRn)
-              -> TcRhoType
+              -> ExpRhoType
               -> TcM (GRHSs GhcTc (LHsExpr GhcTc))
 
 tcMatchesFun :: Located Name