Merge is not that useful with indexed types

[augustobmoura]

When using Merge with an indexed type on either left or right hand all the well-known properties are lost. This is because keyof of an indexed type always returns 'string | number', because every key is possible in that object.

Example:

type Example = {
  foo: string,
  bar: symbol,
  [key: string]: unknown,
}

type Foo = {
  bar: Date,
}

/*
  We would expect something like:
  {
    foo: string,
    bar: Date,
    [key: string]: unknown,
  }
  
  but it actually returns:
  {
    [key: string]: unknown,
    [key: number]: unknown,
  }
*/
type result = Merge<Example, Foo>

This is a well-known caveat with Typescript that was discussed in a few issues (microsoft/TypeScript#45367, microsoft/TypeScript#31153), the behavior is actually intended.

And Omit indeed loses the well-known keys in the same way:

/*
  Returns:
  {
    [key: string]: unknown,
    [key: number]: unknown,
  }
*/
type omit = Omit<Example, 'foo'>

/*
  Our current implementation of Except on the other hand works fine with indexed types
  This returns:
  {
    bar: Date,
    [key: string]: unknown,
  }
*/
type except = Except<Example, 'foo'>

What I propose is to either change Merge to support indexed types and return what I'm expecting on the example (similar to except), which could be a breaking change for people expecting to lose typing on these cases (but I doubt is a useful behavior), or we can create a new MergeWellKnownKeys that treats indexed types. What do you guys think?

I can work on a PR as soon as we decide on an implementation. It should be easy because we already have an internal type doing the same thing on our company's project

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[sindresorhus]

What I propose is to either change Merge to support indexed types and return what I'm expecting on the example (similar to except)

I agree. That's the expected behavior. I would consider this a bug fix.

[skarab42]

@sindresorhus If you want I can fix this before v3 (#387) because if we change the return to integrate the indexes it becomes a bracking change.

[sindresorhus]

@skarab42 That would be great 👍