It doesn't require that. Flow seems to accept all sorts of variations on the following (including X | X): type X = { foo: string } type Y = { foo: string

It doesn't require that. Flow seems to accept all sorts of variations on the following (including X | X): type X = { foo: string } type Y = { foo: string, bar: string } type Foo = X | Y (Y is a subset of X in that case.)