Template Literal Types derived from the type keys cannot be used as Indexed Access Types in generic contexts

🔎 Search Terms

template literal, indexed access, keyof, key of

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about indexed access.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.1-rc#code/PTAEBUE8AcFMGdQHcCWAXAFqAdgVwLawBOKAxqANayTwB0AUGjLKAIKgC8oA2gIwC6AbkbNQAIU6he3ISLigAwpIDePKpABcOAgCNi-Lb1ABfYU3kARFaAAMhk8Pog2AGxcB7JLAAmoFNm9YAA8fUABDUlIERHMEBljXDy9fLm5Wbht+ABo2bgByGzzs3Lx8PSJi9IADABJlUvLjKuKxDJb8wvaG-RzW2vrdYibihTac0YKi8e5uiun+2eGcizHQFcn+WWcLFHgwt09Q-0CQ3wio+BjmOjkWHb2D5Mk07ng0EmwAc3a3j+-p37+f5rV7vIHFFazCHcBaDIjDWS3UAAOXcaFY2AAkpdcLAADzgUDBNCwAKIdw6ABWsFIaAAfJJwDC6up3AAzCDDMyibHwXEEolBElk0AU6m0hlcJn9VkcwkAMlAAApAV9QAAfbRlYga0A6FCffxoXU6dzuFywMLYXV4Ny63ABWBs-w+ACUXKRCnc+GgETQvP5hOJpO85KpNPpjOZygAokFSC5cIE8bKIDl4JAyua6VygA

💻 Code

// Types with numeric keys.
type A = [1];
type B = 1[];
type C = { [key: number]: 1 };
type D = { 0: 1 };

// Allowed indexed access types.
type Allowed = [A[0], A['0'], A[number], A[`${number}`], B[0], B['0'], B[number], B[`${number}`], C[0], C['0'], C[number], C[`${number}`], D[0], D['0']];
// Disallowed indexed access types.
type Disallowed = [A[string], B[string], C[string], D[string], D[number], D[`${number}`]];

type NotAnIssue<T extends object> = T[`${keyof T}`];
type Issue<T extends object> = T[`${keyof T & (string | number | bigint | boolean | null | undefined)}`];
type CompactIssue<T extends object> = T[`${Exclude<keyof T, symbol>}`];

🙁 Actual behavior

The template type literal generated from keyof T cannot be used as an indexed access type for T.

🙂 Expected behavior

The template type literal generated from keyof T or its restriction can be used as an indexed access type for T, since it is restricted to be a stringified version of the key type, which should behave exactly the same as the key type in this context.

Additional information about the issue

All object types with numeric keys can be accessed with the string version of that key. However, the compiler does not acknowledge this fact in the case of generics with proper restrictions on the type.

Using `${keyof T}` directly is prohibited because template literal types cannot be constructed from symbol types. However, if the type is restricted to the allowed string | number | bigint | boolean | null | undefined, TypeScript is still refusing to use keyof T for indexed access.

The issue does not happen with definite (non-generic) types and the issue also does not occur if `${Exclude<keyof T, symbol>}` is replaced with Exclude<keyof T, symbol> (which is still a problem for other use cases).

リンクされた TypeScript Playground でジェネリックなインデックスアクセスのケースを再現し、issue にある許可済みの例および非ジェネリックな例と比較してください。keyof T から派生するテンプレートリテラル型をコンパイラーがどのように扱うかを追跡します。制限されたテンプレートリテラル形式がインデックスアクセス型として受け入れられ、列挙された許可されないケースを再び許可しないことを確認できれば完了です。