ignore type of overwritten object spread properties?

[jedwards1211]

The following function is guaranteed to set the name property to a defined value, but Flow complains about the ...input spread because its name property is optional. Maybe the type of name from ...input should be ignored since it is overwritten by the name in the object literal?

Code Try flow link

type Optional = {name?: ?string, foo: number}
type Required = {name: string, foo: number}

function normalize(input: Optional): Required {
  return {
    ...input,
    name: input.name || 'unnamed',
  }
}

Errors

5:   return {
            ^ Cannot return object literal because null or undefined [1] is incompatible with string [2] in property `name`.
References:
1: type Optional = {name?: ?string, foo: number}
                           ^ [1]
2: type Required = {name: string, foo: number}

Less performant workaround

The only type-safe workaround I can think of (using object rest spread or similar) incurs a performance penalty:

function normalize({name, ...input}: Optional): Required {
  return {
    ...input,
    name: name || 'unnamed',
  }
}

[ghost]

type Optional = {name: ?string, foo: number}
type Required = {name: string, foo: number}

function normalize(input: Optional): Required {
  return (({
    ...input,
    name: input.name && 'unnamed',
  }:any):Required)
}

[jedwards1211]

@bhandarijiwan yes, that's a reasonable workaround, and I do things like that in all sorts of places... Using any is not truly type-safe in the sense that one could accidentally make mistakes in the type definitions of Optional and Required such that some props are not actually compatible, but Flow would have no way of knowing due to any usage. But oftentimes one still has to use any, and functions like that are the safest way to do it :)

[adimit]

This is a duplicate of #6108.

[goodmind]

Duplicate of #6108