Structural assignability of mapped types when key type is generic

[Igorbek]

TypeScript Version: 2.1.4

Code

type M<K extends string> = { [P in K]: P; };

function f<K extends string>() {
    const a1 = null as any as ({ [P in K]: P; });
    const a2: { [P in K]: P; } = a1;
        //~~ Type '{ [P in K]: P; }' is not assignable to type '{ [P in K]: P; }'. Two different types with this name exist, but they are unrelated.

    // However, if a type definition is used
    const a3 = null as any as M<K>;
    const a4: M<K> = a3;
        //^^ OK
}

// when keyof is used, it works
function g<T>() {
    const a1 = null as any as ({ [P in keyof T]: P; });
    const a2: { [P in keyof T]: P; } = a1;
        //^^ OK
}

Expected behavior:

No error.

Actual behavior:

Error:

Type '{ [P in K]: P; }' is not assignable to type '{ [P in K]: P; }'. Two different types with this name exist, but they are unrelated.
  Type 'P' is not assignable to type 'P'. Two different types with this name exist, but they are unrelated.
    Type 'K' is not assignable to type 'P'.
      Type 'string' is not assignable to type 'P'.

[Igorbek]

Sorry, dup #12832