From 248696f06b21dc7bafaaa6ff784303d189149783 Mon Sep 17 00:00:00 2001
From: Changlin Li <mail@changlinli.com>
Date: Wed, 17 Jul 2024 14:51:56 -0700
Subject: [PATCH] Minimal fix for issue #2241

This fixes https://github.com/elm/compiler/issues/2241

Preliminary testing seems to indicate this isn't as bad of a slowdown as
it may seem. For more details see https://github.com/Zokka-Dev/zokka-compiler/pull/20

This is meant as a reference PR for other people working on the Elm
compiler.
---
 compiler/src/Type/Unify.hs | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/compiler/src/Type/Unify.hs b/compiler/src/Type/Unify.hs
index 837d930e0..2a56a9e34 100644
--- a/compiler/src/Type/Unify.hs
+++ b/compiler/src/Type/Unify.hs
@@ -166,15 +166,20 @@ fresh (Context _ (Descriptor _ rank1 _ _) _ (Descriptor _ rank2 _ _)) content =
 guardedUnify :: Variable -> Variable -> Unify ()
 guardedUnify left right =
   Unify $ \vars ok err ->
-    do  equivalent <- UF.equivalent left right
-        if equivalent
-          then ok vars ()
+    do  occursLeft <- Occurs.occurs left
+        occursRight <- Occurs.occurs right
+        if occursLeft || occursRight
+          then err vars ()
           else
-            do  leftDesc <- UF.get left
-                rightDesc <- UF.get right
-                case actuallyUnify (Context left leftDesc right rightDesc) of
-                  Unify k ->
-                    k vars ok err
+            equivalent <- UF.equivalent left right
+            if equivalent
+              then ok vars ()
+              else
+                do  leftDesc <- UF.get left
+                    rightDesc <- UF.get right
+                    case actuallyUnify (Context left leftDesc right rightDesc) of
+                      Unify k ->
+                        k vars ok err
 
 
 subUnify :: Variable -> Variable -> Unify ()