From 3c0d51b3edc393c79071e1adff6767981e7f9424 Mon Sep 17 00:00:00 2001
From: Daniel Rosenwasser <DanielRosenwasser@users.noreply.github.com>
Date: Mon, 1 Nov 2021 09:17:03 +0000
Subject: [PATCH] Check for type parameters on the original type when asking
 for the type argument constraint.

---
 src/compiler/checker.ts | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 7dddeaac50788..9a8e9eb1ee868 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -34729,7 +34729,8 @@ namespace ts {
         function getTypeArgumentConstraint(node: TypeNode): Type | undefined {
             const typeReferenceNode = tryCast(node.parent, isTypeReferenceType);
             if (!typeReferenceNode) return undefined;
-            const typeParameters = getTypeParametersForTypeReference(typeReferenceNode)!; // TODO: GH#18217
+            const typeParameters = getTypeParametersForTypeReference(typeReferenceNode);
+            if (!typeParameters) return undefined;
             const constraint = getConstraintOfTypeParameter(typeParameters[typeReferenceNode.typeArguments!.indexOf(node)]);
             return constraint && instantiateType(constraint, createTypeMapper(typeParameters, getEffectiveTypeArguments(typeReferenceNode, typeParameters)));
         }