Issue with constraint propagation

[lpw25]

A bug report from @jordwalke:

module type OneSig = sig
  type t
  val oneValue: t -> string
end

module type TwoSig = sig
  type t
  val twoValue: t -> int
end

let hasBothOneAndTwo {OneImpl: OneSig} {TwoImpl: TwoSig with type t = OneImpl.t} o =
  let twoValue = string_of_int (TwoImpl.twoValue o) in
  let oneValue = OneImpl.oneValue o in
    oneValue ^ twoValue

implicit module HasBothForInt = struct
  type t = int
  let create () = 0
  let oneValue t = string_of_int t
  let twoValue t = t
end

implicit module HasBothForFloat = struct
  type t = float
  let create () = 0.0
  let oneValue t = string_of_float t
  let twoValue t = int_of_float t
end

let _ = hasBothOneAndTwo 4

gives the error:

Error: Ambiguous implicit OneImpl: HasBothForFloat and HasBothForInt are both solutions.

This is because functions with types like:

  val hasBothOneAndTwo :
    {OneImpl : OneSig} ->
      {TwoImpl : TwoSig with type t = OneImpl.t} ->
        TwoImpl.t -> string

are not handled correctly. Constraints generated by the TwoImpl.t
parameter are not propogated to the OneImpl implicit parameter. Since
the implicit parameters are currently searched left-to-right (at some
point we will make the search order-independent anyway, and then this
bug will be less noticable) the search for OneImpl fails as ambiguous.

There is a work-around, functions with types like:

  val hasBothOneAndTwo :
    {OneImpl : OneSig} ->
      {TwoImpl : TwoSig with type t = OneImpl.t} ->
        OneImpl.t -> string

do not suffer from this problem, so adding a type annotation fixes the issue:

let hasBothOneAndTwo {OneImpl: OneSig} {TwoImpl: TwoSig with type t = OneImpl.t} (o : OneImpl.t) =
  let twoValue = string_of_int (TwoImpl.twoValue o) in
  let oneValue = OneImpl.oneValue o in
    oneValue ^ twoValue