TypeScript: Lookup type on `this` unexpectedly takes `true` branch of conditional type

• Asked on Gitter
• Tried my best to search existing issues
• Tried questioning my understanding, but still the behavior feels weird to me
• Finally, I am forced to create this issue (though I know, the issues list is too big)

TypeScript Version: v3.8.3 and Nightly Search Terms: Generics within class body, Generics in class body

Code

type IsString<T extends any, K extends keyof T> = T[K] extends string ? T[K] : never

class Foo {
  public username!: string
  public age!: number
  public dob!: { [key: string]: any }

  public getStringAttr<T, K extends keyof T>(this: T, key: K): IsString<T, K> {
    return {} as unknown as IsString<T, K>
  }

  public someInternalFunction() {
    this.getStringAttr('dob')
  }
}

new Foo().getStringAttr('dob')

The above is just a dummy code to reproduce the issue with minimum effort.

• The class Foo has a method getStringAttr, which accepts the name of the class property and returns it back if it is a string, or never if it is not a string

Expected behavior:

Regardless of where the getStringAttr method is called, it should have consistent behavior.

Actual behavior:

• Calling getStringAttr by creating a new class instance works fine
• Calling getStringAttr from a different class method (within the class body) makes it infer incorrect types
• The getStringAttr('dob') should return never in both cases. But within the class body it returns string.
• The getStringAttr('age') should return never in both cases and it does so.

Playground Link: https://www.typescriptlang.org/play/index.html?ssl=1&ssc=1&pln=17&pc=31#code/C4TwDgpgBAkgzgZWAJwJYDsDmAeAKlCAD2AnQBM4oBDdEAGigGkDjSKoBrCEAewDMouAHxQAvIIDajALosS5SnBQZMUAPySZUAFxR0EAG4RkAWABQ5gMYAbKnEoAxHjygBvc1ChgArgCNrqJZQ3nDG6FQAthAAhLpKaFgeXn4BQVSYMbro3hG+xkk+-oFQZDy+sW5QElwgccpY0ro0IFAAvuYFKcUZwEgJmACCwCh4DMxE8uw1-IJCABTAABaocLq4DDW6jACUuvB9KqNMIu5mnp7IEMDeyOhurdSU3ugc6DwA7nd2sIj1OOvHJLtCxnZJFIJwHhRGDoEi3KjWBzPSzAVA8dBzbZuJKeJYrAB0PQOWCGKDmAHJSr5ydsgeZgeZ9O8oE4eJjCVdiYNhsgKVSaUA

Related Issues: Cannot find any