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

🔎 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.

Start with the linked TypeScript Playground reproduction and compare the failing generic indexed-access branch with the working variant that adds an array type alias. Read the behavior discussed in PR 48837, then verify the original example no longer reports the constraint error while preserving the expected inferred mapped type behavior.