While working on mapped types (#157), I found the following

interface X {
    a: 2,
    A: 4,
    b: 6
}

type UT<Type> = {
    [Property in keyof Type as Capitalize<string & Property>]: Type[Property]
};

declare let x: UT<X>;
x.A

x.A: 2 | 4

This is difficult because Property contribution is set inside

The argument could be fixed via

• CovariantContribution::CaseInsenstive
• Which specialised to intrinsic TypeId::CASE_INSENSITIVE alias type (similar to existing intrinsics)

The actual lookup is a bit more complex

• get_property_unbound with PropertyKey where it can be an or (or or-like through a big or) should collect possibles
  • Does this collect things correctly? Is some information lost and the parameter made bigger?
• slice_matches should have a case insensitive boolean parameter?

Also

• function x<T>(u: Uppercase<T>): T. x("hi"): CaseInsensitive<"hi">
• Subtyping