String Indexer Assignable to Generic Mapped Type #28798

function tester<K extends 'a'>(x: Record<K, number>, y: { [x: string]: number }) {
    x = y; // no error: not expected
    y = x; // no error: expected
}

function tester2(x: Record<'a', number>, y: { [x: string]: number }) {
    x = y; // error: expected
    y = x; // no error: expected
}

// A source type T is related to a target type { [P in Q]: X } if Q is related to keyof T and T[Q] is related to X.
if (!isGenericMappedType(source) && isRelatedTo(getConstraintTypeFromMappedType(target), getIndexType(source))) {
    const indexedAccessType = getIndexedAccessType(source, getTypeParameterFromMappedType(target));
    const templateType = getTemplateTypeFromMappedType(target);
    if (result = isRelatedTo(indexedAccessType, templateType, reportErrors)) {
        return result;
    }
}

filterType(getIndexType(source), t => !(t.flags & (TypeFlags.String | TypeFlags.Number)
// and symbol??