Typescript 4.7 thinks keyof interface may contain a symbol

[bschlenk]

Bug Report

🔎 Search Terms

Implicit conversion symbol fail runtime

🕗 Version & Regression Information

• This changed between versions 4.6.* and 4.7.*

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Property<T> {
  name: keyof T
}

interface ThingA {
  a: string
  b: string
}

interface ThingB {
  c: string
  d: string
}

function Filter<T extends ThingA | ThingB>(properties: Property<T>[]) {
  return properties.map((property) => `input-${property.name}`) // <-- Implicit conversion of a 'symbol' to a 'string' will fail at runtime. Consider wrapping this expression in 'String(...)'.(2731)
}

🙁 Actual behavior

Typescript thinks that property.name here may be a Symbol. I guess maybe this makes sense since T extends ThingA | ThingB wouldn't prohibit a ThingC interface that extends ThingA with a symbol property. But I'd expect this new error to pop up in a lot more places in my codebase, but I only see it in this one spot.

I'm not sure in what circumstances this error is triggered, or how to work around it. I'd rather not add String(...) around the property because that's unnecessary extra runtime code.

🙂 Expected behavior

Because this doesn't fail in TS < 4.7, I'd expect it not to suddenly start failing.

[MartinJohns]

Related: #49071 (was mentioned there)
Potentially fixed by #49107.

The suggestion by Daniel Rosenwasser was to write string & keyof T.

[RyanCavanaugh]

This is a correct error because this function crashes on legal inputs:

const m: { a: string, b: string, [Symbol.iterator]: ""} = { a: "", b: "", [Symbol.iterator]: ""};
Filter<typeof m>([{name: Symbol.iterator}]);

Uncaught (in promise) TypeError: Cannot convert a Symbol value to a string
    at <anonymous>:2:59
    at Array.map (<anonymous>)
    at Filter (<anonymous>:2:23)
    at <anonymous>:5:1

[bschlenk]

Thanks @RyanCavanaugh that makes sense! And thanks for the string & keyof T suggestion Martin, that works for me.