Generic indexed access on a substitution type with an array constraint resolves incorrectly

リンクされている TypeScript Playground の再現コードから始め、失敗するジェネリックなインデックスアクセス分岐と、配列型エイリアスを追加した動作するバリアントを比較します。PR 48837 で説明されている挙動を読み、そのうえで、元の例が制約エラーを報告しなくなり、期待される推論された mapped type の挙動が維持されていることを確認します。

🔎 Search Terms

indexed access substitution type array constraint

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAMglsCAnAhgGwCrmgXigZ2CTgDsBzKAHyhIFcBbAI2SqkYHt20IUSBuAFChIUAGrpaEAOIRgiJAB5xaSVhEQAHohIATfLATJ0a3AfnHsAPih4AFAEob15auyCh2KAGUAxgAsIehQbb39AlABBJFQQVl8AoIB5RgArCB9gVnhzTDcPEXjwqJiQ4pQQBUKk1PTM6irI6PKsw1RcyEtBYWgG5LSMkIBvASgoAG0AawgQAC4CIlIyAF053pqB+rCgstjqbKN2iEEAX3duqABJEgAzZBcIfAAxJHZ6BsqoTW09UISUazwXk+Wggun0+zaJhGUAA-GIJNJZPJKpZoatgd99A0dtC4cNRgTxgBpKCkKBTEDsa7eFaXG53BFPF5vLYoSpjIlLTrQ0bHNHeDGgn5rfrAXFQfGE4mkkjk6ZUmlzK63JD3Jmvd5eDlcwSEvmjOYkCAAN2QgiAA

💻 Code

type LiteralType = string | number | boolean;
type ValueGetter<ValueType extends LiteralType = LiteralType> = () => ValueType;

type Schema = SchemaArray | SchemaObject | LiteralType;
type SchemaArray = Array<SchemaObject | SchemaArray | LiteralType>;
type SchemaObject = {
  [key: string]: SchemaObject | SchemaArray | LiteralType;
};

type InferValuesFromSchema<S extends Schema> = S extends LiteralType
  ? ValueGetter<S>
  : S extends SchemaArray
  ? {
      [K in keyof S]: InferValuesFromSchema<S[K]>;
    }
  : S extends SchemaObject
  ? {
      [K in keyof S]: InferValuesFromSchema<S[K]>;
    }
  : never;

🙁 Actual behavior

an error is reported:

Type 'S[K]' does not satisfy the constraint 'Schema'.
  Type 'S[keyof S]' is not assignable to type 'Schema'.
    Type 'S[string] | S[number] | S[symbol]' is not assignable to type 'Schema'.
      Type 'S[string]' is not assignable to type 'Schema'.
        Type 'S[string]' is not assignable to type 'SchemaObject'.
          Type 'S[keyof S]' is not assignable to type 'SchemaObject'.
            Type 'S[K]' is not assignable to type 'SchemaObject'.
              Type 'S[keyof S]' is not assignable to type 'SchemaObject'.
                Type 'S[string] | S[number] | S[symbol]' is not assignable to type 'SchemaObject'.
                  Type 'S[string]' is not assignable to type 'SchemaObject'.(2344)

🙂 Expected behavior

There should be no error, S[K] should satisfy Schema constraint (at least since https://github.com/microsoft/TypeScript/pull/48837 )

Additional information about the issue

A variant with an additional type alias for that array branch works OK: TS playground.