Relations based on variance markers do not take into account parameters used as key types.

TypeScript Version: 3.3

Code

type Record2<K extends keyof any, T> = {
    [P in K]: T;
};

function defaultRecord(x: Record<'a', string>, y: Record<string, string>) {
    x = y; // no error, but error expected.
}

function customRecord(x: Record2<'a', string>, y: Record2<string, string>) {
    x = y; // no error, but error expected.
}

function mixed1(x: Record2<'a', string>, y: Record<string, string>) {
    x = y; // error
}

function mixed2(x: Record<'a', string>, y: Record2<string, string>) {
    x = y; // error
}

Playground Link: link

Related Issues: #28798