Fix #13819 by refactoring TypeEqOrigin.uo_thing

The uo_thing field of TypeEqOrigin is used to track the
"thing" (either term or type) that has the type (kind) stored
in the TypeEqOrigin fields. Previously, this was sometimes a
proper Core Type, which needed zonking and tidying. Now, it
is only HsSyn: much simpler, and the error messages now use
the user-written syntax.

But this aspect of uo_thing didn't cause #13819; it was the
sibling field uo_arity that did. uo_arity stored the number
of arguments of uo_thing, useful when reporting something
like "should have written 2 fewer arguments". We wouldn't want
to say that if the thing didn't have two arguments. However,
in practice, GHC was getting this wrong, and this message
didn't seem all that helpful. Furthermore, the calculation
of the number of arguments is what caused #13819 to fall over.
This patch just removes uo_arity. In my opinion, the change
to error messages is a nudge in the right direction.

Test case: typecheck/should_fail/T13819

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/ghci/RtClosureInspect.hs b/compiler/ghci/RtClosureInspect.hs
index 785513b3b6..263aeba7e9 100644
--- a/compiler/ghci/RtClosureInspect.hs
+++ b/compiler/ghci/RtClosureInspect.hs
@@ -637,7 +637,7 @@ addConstraint actual expected = do
       discardResult $
       captureConstraints $
       do { (ty1, ty2) <- congruenceNewtypes actual expected
-         ; unifyType noThing ty1 ty2 }
+         ; unifyType Nothing ty1 ty2 }
      -- TOMDO: what about the coercion?
      -- we should consider family instances
 
@@ -1186,7 +1186,7 @@ congruenceNewtypes lhs rhs = go lhs rhs >>= \rhs' -> return (lhs,rhs')
                (_, vars) <- instTyVars (tyConTyVars new_tycon)
                let ty' = mkTyConApp new_tycon (mkTyVarTys vars)
                    rep_ty = unwrapType ty'
-               _ <- liftTcM (unifyType noThing ty rep_ty)
+               _ <- liftTcM (unifyType Nothing ty rep_ty)
         -- assumes that reptype doesn't ^^^^ touch tyconApp args
                return ty'