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#53413 shouldn't have fixed anything, so I do wonder what the heck I did...

@jakebailey the circularity was caused by subtype reduction - ur PR avoids it in certain cases and thus it accidentally~ fixed this issue

I re-checked and at the time the circularity was encountered by the subtype reduction we can see those types when we get to your optimization:

typeToString(getTypeOfSymbol(leftProp)) // Foo | Box
typeToString(rightType) // Foo | Box | undefined

So it makes sense that we don't get to the circularity now since u avoid that subtype reduction based on this check:

result.links.type = leftTypeWithoutUndefined === rightTypeWithoutUndefined ? leftType : getUnionType([leftType, rightTypeWithoutUndefined], UnionReduction.Subtype);

I'd call it - "accidental but good" :P