Symbol is incompatible with index signature

[DeTeam]

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:

• TS2536: Type 'unique symbol' cannot be used to index type 'T'. #21912
• Allow indexing with symbols #1863
• Property '[mySymbol]' is not assignable to string index type #20146

[yortus]

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'

[mhegazy]

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

[weswigham]

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