Symbol is incompatible with index signature

I'm trying to use both the index signature in my interface and some symbol fields. They don't seem to work very well together.

Code

export const KEY = Symbol('KEY')

export interface Tree {
  [KEY]?: string
  [name: string]: Tree
}

// Here I get a type problem
export const tree: Tree = {
  'a': {
    [KEY]: 'value1',

    'x': {
      [KEY]: 'value2'
    }
  },
  'b': {
    [KEY]: 'value3'
  },
  'c': {
    [KEY]: 'value4'
  }
}

Expected behavior:

I think there're two options here:

Either complain about things on the type level (like if one'd use key (just a string field name) instead of a symbol [KEY])
Or allow such usage together with access later const s: Tree = tree['c']; const s: string = tree['c'][KEY]

Actual behavior:

Property '[KEY]' is incompatible with index signature.

Playground Link:

Related Issues:

I think that maybe the following ones are related in a way:

I think #21217 was meant to fix this, but this shows there is still a problem. Simpler repro:

export const SYM = Symbol('a unique symbol');

export interface I {
  [SYM]: 'sym';
  [x: string]: 'str';
}

let a: I = {[SYM]: 'sym'}; // ERROR: 'sym' is not assignable to 'str'
let b: I = {[SYM]: 'str'}; // ERROR: 'str' is not assignable to 'sym'

Wesley Wigham (@weswigham)

Wesley Wigham (@weswigham) i would have expected this to be fixed by #22339, but does not seem to be the case..

indexTypesRelatedTo inside isRelatedTo doesn't filter out symbol-keyed properties for its eachPropertyRelatedTo check, which is the actual root issue for this one.

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Symbol is incompatible with index signature #21962

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Symbol is incompatible with index signature #21962

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yortus commented on Feb 16, 2018

mhegazy commented on Mar 8, 2018

weswigham commented on Mar 8, 2018

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