Lookup on object of indeterminate type

I'm trying to figure out why this doesn't compile.

type A() =
  member __.M(f:DateTime -> seq<int>) 开发者_如何学Go= ()
  member __.M(f:DateTime -> obj) = ()

let a = A()
a.M(fun d -> seq [d.Year]) // 'd' is indeterminate type

If I remove the second overload or add a type annotation to d, it does. Is it because some aspect of overload resolution occurs prior to type checking?

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Yes, basically overloads make type inference hard. In this case, it seems like you're hoping that the compiler will do some sort of generalization of the types DateTime -> seq<int> and DateTime -> obj to get DateTime -> ? and then proceed from there, but I don't think it ever does that kind of "anti-unification".