Union validate wrongly validates object when the runtypes have the same property and one of the properties is a Union

[jsmith]

Sorry for the long title, but i think I found an issue where the type check wrongly validates an object. Reproduction link to a codesandbox but here is the code:

import { Record, Literal, Union } from "runtypes";

const FooBar = Record({
  foo: Literal("bar"),
});

const AB = Record({
  type: Union(Literal("A"), Literal("B")),
  foobar: FooBar,
});

const C = Record({
  type: Literal("C"),
  foobar: FooBar,
});

const ABC = Union(AB, C);

const input = {
  type: "C",
  foobar: {},
};

console.log(ABC.guard(input)); // returns true

The cause seems to be type: Union(Literal("A"), Literal("B")). If you split AB into A and B, it works as expected since the validator doesn't even look at A or B.

The validator first checks AB. When it does this, it checks foodbar and adds {} + FooBar it to the VisitedState. Next, when it checks C, the type property matches and then when it checks foorbar, {} + FooBar is already in the VisitedState so the object type checks successfully.

I'm not sure the best solution. Maybe the code shouldn't assume that if value and A are in VisitedState, it has been validated successfully? Thanks for the help!