error: Lookup on object of indeterminate type based on information prior to this program point. A type annotation may be needed prior to this program point to constrain the type of the object. This may allow the lookup to be resolved.

test.fs

type Foo() =
    member __.bar = "bar"

type Bar() as self =
    let foo = Foo()
    do
        printf "%A" foo.bar
        printf "%A" self.Foo.bar
    member __.Foo = foo

8行目で「Lookup on object of indeterminate type based on information prior to this program point. A type annotation may be needed prior to this program point to constrain the type of the object. This may allow the lookup to be resolved.」というエラーが出る。Fooに型ヒントを付けるとエラーが消える。解せない。