AA.t and BB.t are two distinct abstract types, so they can't be unified,

you should export the type definitions in AA and BB : 

module AA : A1 with type t = int = struct 
    type t = int
    let test f1 f2 = true
end

module BB : A2 with type t = int = struct
    type t = int
    let toast f1 f2 = false
end

module C = Make (AA) (BB)


damien


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