From 947b6d874004a2e61c56ff74d409405db0782e13 Mon Sep 17 00:00:00 2001
From: Sam Vervaeck <samvv@pm.me>
Date: Sun, 19 Mar 2023 15:04:47 +0100
Subject: [PATCH] Manually filter out type variables in type scheme that do
 nothing

---
 src/checker.ts | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/checker.ts b/src/checker.ts
index e37365223..0835dc756 100644
--- a/src/checker.ts
+++ b/src/checker.ts
@@ -838,10 +838,12 @@ class Forall extends SchemeBase {
     public type: Type,
   ) {
     super();
-    if (typeVars instanceof TVSet) {
-      this.typeVars = typeVars;
-    } else { 
-      this.typeVars = new TVSet(typeVars);
+    this.typeVars = new TVSet();
+    const allowed = new TVSet(type.getTypeVars());
+    for (const tv of typeVars) {
+      if (allowed.has(tv)) {
+        this.typeVars.add(tv);
+      }
     }
   }