From 6d33d8797b5918ebc8a7ed3e1dea5ffcc60057e3 Mon Sep 17 00:00:00 2001
From: Daniel Rosenwasser <DanielRosenwasser@users.noreply.github.com>
Date: Mon, 1 Nov 2021 11:41:43 -0700
Subject: [PATCH] Check for type parameters on original type when getting type
 argument completions (#46614)

* Add failing test.

* Check for type parameters on the original type when asking for the type argument constraint.

* Update tests/cases/fourslash/stringLiteralTypeCompletionsInTypeArgForNonGeneric1.ts
---
 src/compiler/checker.ts                                |  3 ++-
 ...ingLiteralTypeCompletionsInTypeArgForNonGeneric1.ts | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)
 create mode 100644 tests/cases/fourslash/stringLiteralTypeCompletionsInTypeArgForNonGeneric1.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 7dddeaac507..9a8e9eb1ee8 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)));
         }
diff --git a/tests/cases/fourslash/stringLiteralTypeCompletionsInTypeArgForNonGeneric1.ts b/tests/cases/fourslash/stringLiteralTypeCompletionsInTypeArgForNonGeneric1.ts
new file mode 100644
index 00000000000..7c1a11889a6
--- /dev/null
+++ b/tests/cases/fourslash/stringLiteralTypeCompletionsInTypeArgForNonGeneric1.ts
@@ -0,0 +1,10 @@
+/// <reference path="./fourslash.ts" />
+
+////interface Foo {}
+////type Bar = {};
+////
+////let x: Foo<"/*1*/">;
+////let y: Bar<"/*2*/">;
+
+verify.completions({ marker: test.markers() });
+