typeguard else statement is not working properly

[ATheCoder]

Bug Report

🔎 Search Terms

Typeguard else not working

Type guard not working

🕗 Version & Regression Information

• I first had this issue on version 3.9.5 but I also tested it on the playground nightly and it still happens.

⏯ Playground Link

Playground link with relevant code

💻 Code

const regularNameToUnicodeMap = {
  Home: String.fromCharCode(0xf262),
  Plugin: String.fromCharCode(0xf261),
  Tree: String.fromCharCode(0xf25f),
  Schema: String.fromCharCode(0xf25e),
};

const solidNameToUnicodeMap = {
  Search: String.fromCharCode(0xf002),
};

interface IconProps<T extends 'regular' | 'solid'> {
  type: T;
  icon: T extends 'regular'
    ? keyof typeof regularNameToUnicodeMap
    : keyof typeof solidNameToUnicodeMap;
}

type RegularIcon = keyof typeof regularNameToUnicodeMap;
type SolidIcon = keyof typeof solidNameToUnicodeMap;

const isRegular = (
  type: 'regular' | 'solid',
  icon: RegularIcon | SolidIcon,
): icon is RegularIcon => {
  return type === 'regular';
};

const P = <T extends 'solid' | 'regular'>(props: IconProps<T>): any => {
        if(isRegular(props.type, props.icon)){
                return regularNameToUnicodeMap[props.icon]
        } else {
                return solidNameToUnicodeMap[props.icon] //Error happens here.
        }
}

🙁 Actual behavior

I get an error:

Type 'T extends "regular" ? "Home" | "Plugin" | "Tree" | "Schema" : "Search"' cannot be used to index type '{ Search: string; }'

🙂 Expected behavior

There shouldn't be an error because according to the docs typescript should know that when the type guard returns false icon is of type SolidIcon.

[RyanCavanaugh]

Given how narrowing and generics interact today, we don't have any way to represent the type of props.icon in the else branch without negated types; see #29317

[sladiri]

Hi, is this the same problem?
I have a very minimal example: https://stackblitz.com/edit/typescript-s2c1jm?file=index.ts

And this looks like a variation? I hoped, I found a workaround: https://stackblitz.com/edit/typescript-itubna?file=index.ts

I guess you can always use switch cases:
https://stackblitz.com/edit/typescript-ut4l46?file=index.ts