Type-casting issue when providing a generic function as argument. about v HOT 2 OPEN

println(v.generic_new(math.abs)) // error

math_abs := fn (s f64) f64 { return math.abs(s) }
println(v.generic_new(math_abs)) // fix with new function

// vlang test, reduced for git:
// - vector operations (example) build with an interface
// - derive functions for a group of basic types
// questions:
// - how to use interface methods withou an additional cast
// - where are casts or generic types missing

module main

// scalar, basic type
type Scalar = f64

// basic function, not build as method, needed as arg
fn add_scalar(s1 Scalar, s2 Scalar) Scalar {
	return s1 + s2
}

// vector, interface an derived functions
interface Vector {
	libs (fn (Scalar, Scalar) Scalar) Scalar
//	lib2 (Vector, fn (Scalar, Scalar) Scalar) Vector
}

// derived method 1
fn (v Vector) sum() Scalar {
	return v.libs(add_scalar)
}

/* // derived method 2
fn (v1 Vector) add(v2 Vector) Vector {
	return v1.lib2(v2, add_scalar)
}
*/

// vec3, representation of the interface
type Vec3 = struct {
	x Scalar
	y Scalar
	z Scalar
}

// basic method a, safisfies the interface type
fn (v Vec3) libs(f fn (Scalar, Scalar) Scalar) Scalar {
	return f(v.x, f(v.y, v.z))
}

/* // interface method b, safisfies the interface type
fn (v1 Vec3) lib2(v2 Vec3, f fn (Scalar, Scalar) Scalar) Vec3 {
//fn (v1 Vec3) lib2(v2 Vector, f fn (Scalar, Scalar) Scalar) Vector {
	return Vec3{f(v1.x, v2.x), f(v1.y, v2.y), f(v1.z, v2.z)}
}
*/

fn main() {
	println('### start')
	
	// define a vector
	v := Vec3{1, 2, 3}
	
	// check if "Vec3" satifies the interface "Vector" -> ok
	println(Vector(v))
	
	// try method of type with fn as arg -> ok
	println(v.libs(add_scalar))
	
	// try methods of interface -> explicit cast needed, why?
//	println(v.sum()) // should work
	println(Vector(v).sum()) // but only works with a cast
	
	// try methods of interface -> error
	// - "Vec3" as args produce "unknown method Vec3.sum/add"
	// - "Vector" as args produce "no field name x/y/z"	
//	println(v.add(v))
	
	// the goal, concat
//	println(v.add(v).add(v))
	
	println('### end')
}