From 83b47715e6e468163eafdd96c7b9cc332cf9e887 Mon Sep 17 00:00:00 2001
From: Josh Goldberg <git@joshuakgoldberg.com>
Date: Mon, 6 Feb 2023 04:53:18 -0500
Subject: [PATCH 1/2] Added failing test for mapped type

---
 src/types/utilities.test.ts | 26 ++++++++++++++++++++++++++
 src/types/utilities.ts      |  7 +++++--
 2 files changed, 31 insertions(+), 2 deletions(-)
 create mode 100644 src/types/utilities.test.ts

diff --git a/src/types/utilities.test.ts b/src/types/utilities.test.ts
new file mode 100644
index 00000000..5d7efc8b
--- /dev/null
+++ b/src/types/utilities.test.ts
@@ -0,0 +1,26 @@
+import * as ts from "typescript";
+import { describe, expect, it } from "vitest";
+
+import { createSourceFileAndTypeChecker } from "../test/utils";
+import { isPropertyReadonlyInType } from "./utilities";
+
+describe("isPropertyReadonlyInType", () => {
+	it("does not crash when the type is a mapped type parameter extending any", () => {
+		const { sourceFile, typeChecker } = createSourceFileAndTypeChecker(`
+            type MyType<T> = {
+                [K in keyof T]: 'cat' | 'dog' | T[K];
+            };
+            type Test<A extends any[]> = MyType<A>;
+        `);
+		const node = sourceFile.statements.at(-1) as ts.TypeAliasDeclaration;
+		const type = typeChecker.getTypeAtLocation(node);
+
+		expect(
+			isPropertyReadonlyInType(
+				type,
+				ts.escapeLeadingUnderscores("length"),
+				typeChecker
+			)
+		).toBe(false);
+	});
+});
diff --git a/src/types/utilities.ts b/src/types/utilities.ts
index c7437993..d3a97045 100644
--- a/src/types/utilities.ts
+++ b/src/types/utilities.ts
@@ -92,8 +92,11 @@ function isReadonlyPropertyFromMappedType(
 		!/^__@[^@]+$/.test(name as string)
 	)
 		return declaration.readonlyToken.kind !== ts.SyntaxKind.MinusToken;
-	return isPropertyReadonlyInType(
-		(type as unknown as { modifiersType: ts.Type }).modifiersType,
+
+	const { modifiersType } = type as { modifiersType?: ts.Type };
+
+	return /* modifiersType && */ isPropertyReadonlyInType(
+		modifiersType!,
 		name,
 		typeChecker
 	);

From d64c9be7a8bfc18cbdc1e504271c10e328536809 Mon Sep 17 00:00:00 2001
From: Josh Goldberg <git@joshuakgoldberg.com>
Date: Mon, 6 Feb 2023 04:57:58 -0500
Subject: [PATCH 2/2] Fixed code to make the test pass

---
 src/types/utilities.ts | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/types/utilities.ts b/src/types/utilities.ts
index d3a97045..5237f4df 100644
--- a/src/types/utilities.ts
+++ b/src/types/utilities.ts
@@ -95,10 +95,8 @@ function isReadonlyPropertyFromMappedType(
 
 	const { modifiersType } = type as { modifiersType?: ts.Type };
 
-	return /* modifiersType && */ isPropertyReadonlyInType(
-		modifiersType!,
-		name,
-		typeChecker
+	return (
+		modifiersType && isPropertyReadonlyInType(modifiersType, name, typeChecker)
 	);
 }