Issue when combining distributive conditional types combined with generic method

[jeroenvervaeke]

TypeScript Version: 3.5.0-dev.20190504

Search Terms:

• distributive conditional types
• condition types
• distributive conditional types generics

Description
I've been trying to make a generic function which receives and object T and receives a string property name of that object T.

I used https://www.typescriptlang.org/docs/handbook/advanced-types.html as an example (section: Distributive conditional types)

I've come up with a solution that works without generics, but when I change the explicit types to a generic type typescript won't compile.

Code

export type TypedPropertyNames<T, P> = { [K in keyof T]: T[K] extends P ? K : never }[keyof T];
export type StringPropertyNames<T> = TypedPropertyNames<T, string>;

interface Test {
  test: string;
}

function non_generic(form: Test, field: StringPropertyNames<Test>): string {
  return form[field]; // This compiles
}

function generic<T>(form: T, field: StringPropertyNames<T>): string {
  return form[field]; // This won't compile
}

Expected behavior:
The generic example compiles

Actual behavior:
The generic example doesn't compile

Playground Link:
Playground

Related Issues:
#30295
#29505
#29662

[jcalz]

For cross-referencing purposes: associated Stack Overflow question

[ahejlsberg]

This is beyond the reasoning capabilities of the type checker. The checker would need to understand that StringPropertyNames<T> is constrained such that T[StringPropertyNames<T>] is constrained to type string. We don't produce such transitive constraints and it would definitely be complex to do so. I'm afraid you'll need a type assertion for this one.

[jcalz]

related: #30728