From e4c7ce6302d4fa57eef3ec85fa467c0aaff866be Mon Sep 17 00:00:00 2001
From: Josh Goldberg <josh@codecademy.com>
Date: Thu, 27 May 2021 14:11:51 -0400
Subject: [PATCH 1/2] Added check for modifiersType existing in a mapped type

---
 util/type.ts | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/util/type.ts b/util/type.ts
index 65cb3c1..b7ddfe7 100644
--- a/util/type.ts
+++ b/util/type.ts
@@ -279,6 +279,10 @@ function isReadonlyPropertyIntersection(type: ts.Type, name: ts.__String, checke
     });
 }
 
+function hasModifiersType(type: ts.Type): type is ts.Type & { modifiersType: ts.Type} {
+    return 'modifiersType' in type;
+}
+
 function isReadonlyPropertyFromMappedType(type: ts.Type, name: ts.__String, checker: ts.TypeChecker): boolean | undefined {
     if (!isObjectType(type) || !isObjectFlagSet(type, ts.ObjectFlags.Mapped))
         return;
@@ -286,6 +290,11 @@ function isReadonlyPropertyFromMappedType(type: ts.Type, name: ts.__String, chec
     // well-known symbols are not affected by mapped types
     if (declaration.readonlyToken !== undefined && !/^__@[^@]+$/.test(<string>name))
         return declaration.readonlyToken.kind !== ts.SyntaxKind.MinusToken;
+
+    if (!hasModifiersType(type)) {
+        return;
+    }
+
     return isPropertyReadonlyInType((<{modifiersType: ts.Type}><unknown>type).modifiersType, name, checker);
 }
 

From 4b8dc9a0a9ced8c3e70b721f4c1b89dfd32f4bfa Mon Sep 17 00:00:00 2001
From: Josh Goldberg <josh@codecademy.com>
Date: Thu, 27 May 2021 14:17:03 -0400
Subject: [PATCH 2/2] Unnecessary curly braces

---
 util/type.ts | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/util/type.ts b/util/type.ts
index b7ddfe7..b615410 100644
--- a/util/type.ts
+++ b/util/type.ts
@@ -291,9 +291,8 @@ function isReadonlyPropertyFromMappedType(type: ts.Type, name: ts.__String, chec
     if (declaration.readonlyToken !== undefined && !/^__@[^@]+$/.test(<string>name))
         return declaration.readonlyToken.kind !== ts.SyntaxKind.MinusToken;
 
-    if (!hasModifiersType(type)) {
+    if (!hasModifiersType(type))
         return;
-    }
 
     return isPropertyReadonlyInType((<{modifiersType: ts.Type}><unknown>type).modifiersType, name, checker);
 }