TypeScript: Incorrect inference when using an optional argument

🔎 Search Terms

Inference, Return, Arguments, Parentheses, Identity Function.

🕗 Version & Regression Information

This reproduces in all available playground versions (tried down to 3.3.3), and also in 5.3.3.

⏯ Playground Link

Link

💻 Code

This is a followup on https://github.com/microsoft/TypeScript/issues/49951 which @Andarist asked me to open a follow-up issue. Also relates to https://github.com/microsoft/TypeScript/issues/47599

Probably better to check the playground for code examples, but here’s the TL;DR:

TYPES DECLERATIONS:

type Schema = Record<string, unknown> // modified from original for the sake of the example, if it doesn't make sense

type StepFunction<
  TSchema extends Schema = Schema,
> = (anything: unknown) => {
  readonly schema: TSchema
  readonly toAnswers?: (keys: keyof TSchema) => unknown
}

function step<TSchema extends Schema = Schema>(
    stepVal: StepFunction<TSchema>,
  ): StepFunction<TSchema> {
    return stepVal
  }

EXAMPLES: Notice the returned object of all functions is the same! The difference is in whether we have the argument for the step function or not. Note that if I do Parameters<typeof myStepValue> even when the argument is missing, it’s inferred correctly (!)

// WORKS: `keys` is inferred based on the `schema`
// - no argument for `step` function
// - no `return` keyword
const workingExample = step(() => ({
  schema: {
    attribute: 'anything',
  },
  toAnswers: keys => {
    // RESULT: `keys` inferred successfully as `attribute`
    type Test = string extends typeof keys ? never : 'true'
    const test: Test = 'true'
    return { test }
  },
}))

// FAILS: `keys` is not inferred based on the `schema`
// - has argument for `step` function
const nonWorkingA = step(_something => ({
  schema: {
    attribute: 'anything',
  },
  toAnswers: keys => {
    // RESULT: `keys` failed to be inferred hence defaults to `string`
    type Test = string extends typeof keys ? never : 'true'
    const test: Test = 'true'
    return { test }
  },
}))

🙁 Actual behavior

Nested value’s function argument cannot be inferred correctly.

If I change the property (toAnswers) from a function to a simple property, there are no inference issues.

🙂 Expected behavior

Nested value function argument should be inferred correctly regardless of the return keyword or declaring the arguments.

Additional information about the issue

Like mentioned above, this is a followup on https://github.com/microsoft/TypeScript/issues/49951 and https://github.com/microsoft/TypeScript/issues/47599