Incorrectly typed argument variant

TypeScript Version: 3.8.3

Search Terms:

Parameters, Array, Overloading, Type inference

Code

type VariantA = [(err: Error, data: string) => any]
type VariantB = [(data: string) => any, (err: Error) => any]
type Args = VariantA | VariantB
 
function fn (...rest: Args): void { /* do something */ }

fn((err, data) => {
  err.stack // Correct: Typed as Error
  data.charAt(0) // Correct: Typed as string
})

fn((data) => {
  data //  --> Error: Mistyped as any, should be string  <--
}, (err) => {
  err.stack // Correct: Typed as Error
})

Expected behavior:

If two functions are passed in (VariantB), the first paramter should be typed as data: string.

Actual behavior:

If two functions are passed in, the first function's parameter is typed as any.

从 issue 中最小的 TypeScript 复现开始，比较一个 callback 和两个 callback 调用时的上下文类型。跟踪 union rest parameter 的类型推断和 overload 处理，然后验证第一个 callback 参数被推断为 string，同时不会导致 Error 和 data 的类型声明发生回归。