Bad type analysis with (very) complex generic types

[sveyret]

TypeScript Version: 3.0.3

Code

I am using the intl-ts library. The following function does not compile because it complains that lang[result] is not known to be executable:

function convertResult<
  T extends Messages &
    { [P in K]: (fieldName: string, p1: P1, p2: P2, p3: P3, p4: P4) => string },
  K extends keyof T,
  P1 = any,
  P2 = any,
  P3 = any,
  P4 = any
>(
  result: K | null,
  params: [P1, P2, P3, P4],
  lang?: Intl<T>,
  fieldName?: string
): boolean | string | null {
  if (result === null) {
    return lang ? null : true
  } else {
    if (lang) {
      return lang[result](fieldName, ...params)
    } else {
      return false
    }
  }
}

Expected behavior:

Typescript should understand that lang[result] is a function.

Actual behavior:

Compiler gives error: “Cannot invoke an expression whose type lacks a call signature. Type '{}' has no compatible call signatures.”

Workaround:

The error can be removed by deleting the Messages inheritance in type T and changing lang from Intl<T> to T.

Playground Link:

A playground example with all needed types is provided here

Related Issues:

You also can refer to the question on StackOverflow

Can you come up with an example that doesn't depend on external frameworks?

[sveyret]

Hi @Andy-MS, full example provided in playground is autonomous and does not need any extra dependency.

Duplicate of #7294

In that example, lang[result] is IntlMessages<T>[K]. We can ignore IntlPrototype (error reproduces without it), and assume that every value of IntlMessages<T> is a MessageFunction<T[P]>, where T[P] is MessageDirect | MessageParams<any, any, any, any, any, any>. So MessageFunction<T[P]> will be either () => string or MessageParams<any, any, any, any, any, any>.

Currently TypeScript doesn't support calling a union. But you can work around this like:

const fn: MessageParams<any, any, any, any, any, any> = lang[result];
return fn(fieldName, ...params)

Note you'll need to increase the length of params by one, which seems correct given that MessageParams takes 6 parameters.

[typescript-bot]

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