Tuples in Swift can usually be directly used as parameters to a function or initializer. It seems that when the parameters to the given function are generics, the compiler can no longer coerce the tuple as parameters. Tested with Swift 2.2 / Xcode 7.3.

GenericTupleParameterCoercing.swift

// Tested using Swift 2.2 / Xcode 7.3
import Swift

// A function and initializer with 2 parameters of explicit types.
func stringTuple(_ lhs: String, _ rhs: String) {
    print("\(lhs), \(rhs)")
}

struct StringTuple {
    init(_ lhs: String, _ rhs: String) {
        print("\(lhs), \(rhs)")
    }
}

// A similar function and initializer, with 2 parameters of the same generic type.
func genericTuple<T>(_ lhs: T, _ rhs: T) {
    print("\(lhs), \(rhs)")
}

struct GenericTuple<T> {
    init (_ lhs: T, _ rhs: T) {
        print("\(lhs), \(rhs)")
    }
}

// Test calling all of the above with a tuple.
let tuple = ("Hello", "World!")

// Both work, as expected.
stringTuple(tuple)
let stringPair = StringTuple(tuple)

// Error: "Missing argument for parameter #2 in call"
genericTuple(tuple)
let genericPair = GenericTuple(tuple)