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, 1 insertions, 1 deletions

diff --git a/testsuite/tests/warnings/should_fail/CaretDiagnostics1.stderr b/testsuite/tests/warnings/should_fail/CaretDiagnostics1.stderr
index aa02b8655a..c2ff69c9ef 100644
--- a/testsuite/tests/warnings/should_fail/CaretDiagnostics1.stderr
+++ b/testsuite/tests/warnings/should_fail/CaretDiagnostics1.stderr
@@ -59,7 +59,7 @@ CaretDiagnostics1.hs:13:7-11: error:
    |       ^^^^^
 
 CaretDiagnostics1.hs:(13,16)-(14,13): error:
-    • Couldn't match expected type ‘Char -> p0’ with actual type ‘()’
+    • Couldn't match expected type ‘Char -> t0’ with actual type ‘()’
     • The function ‘()’ is applied to one value argument,
         but its type ‘()’ has none
       In the expression: () '0'