`IsEqual` identity issues

[trevorade]

There are two self-identity bugs that can pretty simply be fixed: TS Playground

Both of these evaluate to false:

IsEqual<{a: 1} & {a: 1}, {a: 1}>
IsEqual<{a: 1} | {a: 1}, {a: 1}>

The fix is to expand the check a touch:

type IsEqual<A, B> =
    (<G>() => G extends A & G | G ? 1 : 2) extends
    (<G>() => G extends B & G | G ? 1 : 2)
    ? true
    : false;

---

There's also this intersection bug but I'm unaware of a fix: TS Playground

This evaluates to false:

IsEqual<{a: 1} & {b: 2}, {a: 1, b: 2}>

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[tommy-mitchell]

https://github.com/Microsoft/TypeScript/issues/27024#issuecomment-845655557 has a robust solution that seems to solve this (TS Playground link). It's a much longer type though, and seemingly not perfect either.

[tommy-mitchell]

Seemingly, according to Update 1 on this StackOverflow answer, under TypeScript's implementation {a: 1} & {b: 2} and {a: 1, b: 2} aren't actually the same. Not sure if that means they should or shouldn't be equal on the type level, though.

microsoft/TypeScript#48100 is an open issue tracking type equality and its definition. Seems that a recursive Simplify can fix some issues with equality as well.