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

[jack-williams]

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

[ahejlsberg]

The cause of this is #27804. With that PR we started measuring variances for type aliases to object and conditional types. The variance measuring logic determines that K is contravariant and T is covariant in Record<K, T>. This is almost true: An instantiation where K is string is unrelated to an instantiation where K is a literal type (because a string index signature doesn't satisfy an actual property).

Wesley Wigham (@weswigham) Perhaps we should exclude aliases to mapped types from the variance measuring shortcut?

[jack-williams]

Thanks for looking into this Anders Hejlsberg (@ahejlsberg). I guess the difference I'm seeing when using two different aliases is because variance measurements are not shared across different aliases of identical types?

An instantiation where K is string is unrelated to an instantiation where K is a literal type (because a string index signature doesn't satisfy an actual property).

This seems a similar problem to #28798.

I guess the underlying problem is that the semantics of string or number as a value type is different to the semantics as a key type. The logic to relate keys can reuse the logic to relate values by inverting the relationship, which works in most cases, but not when string is involved.

Would another---albeit more costly ---solution be to have a key relation that interprets key types into their value type equivalent before using the value relation?

[ahejlsberg]

Also see #29393 which appears to be an effect of variance measuring for aliased types.

[jack-williams]

I changed the title to something that is hopefully closer to the actual issue. Feel free to change if it is misleading.

My understanding now, after your comments and reading the source, is that a contravariant marker is inferred for the parameter K after:

isRelatedTo(getConstraintTypeFromMappedType(target), getConstraintTypeFromMappedType(source), reportErrors))

but this does not take into account the fact that K is used a key type. Is this correct?

Would it be completely unreasonably to collect uses as key types along with variances, and use that to correctly determine when and how two instantiations of K should be related? I feel that there are some wider issues (#29393) that I don't fully understand, so may well be in completely the wrong ballpark.

Am I right in saying that this is going to be a problem if #26797 lands? Where you could have:

interface Record<K extends string,T> {
 [x: K]: T
}