Spreading identical values with properties and indexers creates errors

[chrbala]

Similar to #8178, but with an additional property. Spreading identical values with properties and indexers creates errors.

export type Trie<T> = {
  [string]: Trie<T>,
  _?: T,
};

const x: Trie<number> = { };
const y: Trie<number> = { };

const z: Trie<number> = { ...x, ...y };

Flow version: 0.117.0

Expected behavior

Two values with identical types should always be spreadable

Actual behavior

Error is thrown:

    11: const z: Trie<number> = { ...x, ...y };
                                           ^ Cannot determine a type for object literal [1]. `Trie` [2] cannot be spread because the indexer string [3] may overwrite properties with explicit keys in a way that Flow cannot track. Can you spread `Trie` [2] first or remove the indexer?
        References:
        11: const z: Trie<number> = { ...x, ...y };
                                    ^ [1]
        9: const y: Trie<number> = { };
                    ^ [2]
        4:   [string]: Trie<T>,
              ^ [3]

• Link to Try-Flow or Github repo: tryflow

[chrbala]

Interestingly, this works if passed into a function, despite having the same effect.

/* @flow */

export type Trie<T> = {
  [string]: Trie<T>,
  _?: T,
};

const x: Trie<number> = { };
const y: Trie<number> = { };

const merge = <T>(a: T, b: T) => ({ ...a, ...b });
const z: Trie<number> = merge(x, y);

Test on tryflow.

[jbrown215]

That's an unsoundness in generic checking.

I have work in the pipeline that allows this, but it exposes some nastier issue somewhere else in the type checker that makes it unshippable. Specifically, it creates an error in a library definition with no references to actual source code.

[geraintwhite]

A similar bug makes it impossible to spread on indexed object into a literal object matching the same type.

tryflow 1

tryflow 2

[SamChou19815]

This is expected behavior that we don't change