I am trying to create a package that solves the kinematics of a closed mechanism. The mechanism type consists of a list of link types that I have also defined parametricly:

```
abstract type Link end
struct BarLink{T} <: Link
r::T # Vector Length
θ::T # Position of link (variable) units of rad
ω::T # Velocity of link, units of rad/s
α::T # Acceleration of link, units of rad/s/s
end
struct Mechanism
n::Integer
input::Integer #index of input link in link vector
variables::Tuple{Integer, Integer} #index of variable links (2)
links::Vector{Link}
end
```

Using Unitful I tried to create a 4-bar Mechanism:

```
bar1 = BarLink{Quantity}(10u"inch", Float64(π)u"rad", 0u"m/m", 0u"m/m") # line throws error
bar2 = BarLink{Quantity}(4u"inch", Float64(2*π/3)u"rad", 45u"rad/s", 0u"rad/s/s")
bar3 = BarLink{Quantity}(10u"inch", deg2rad(47.26)u"rad", 0u"m/m", 0u"m/m")
bar4 = BarLink{Quantity}(12u"inch", deg2rad(295.75)u"rad", 0u"m/m", 0u"m/m")
fourbar = Mechanism(4, 2, (3, 4), [bar1; bar2; bar3; bar4])
```

The error thrown from the initialization of `bar1`

:

```
MethodError: no method matching Unitful.Quantity(::Int64)
```

For reference, I have similar code that works for other subtypes of Number:

```
bar1 = BarLink{Real}(10, π, 0, 0)
bar2 = BarLink{Real}(4, deg2rad(120), 45, 0)
bar3 = BarLink{Real}(10, deg2rad(227.26-180), 0, 0)
bar4 = BarLink{Real}(12, deg2rad(115.75+180), 0, 0)
fourbar = Mechanism(4, 2, (3, 4), [bar1; bar2; bar3; bar4])
```

and with SymPy:

```
@syms r1 r2 r3 r4 θ1 θ2 θ3 θ4 ω2 α2
bar1 = BarLink{Sym}(r1, θ1, 0, 0)
bar2 = BarLink{Sym}(r2, θ2, 0, 0)
bar3 = BarLink{Sym}(r3, θ3, 0, 0)
bar4 = BarLink{Sym}(r4, θ4, 0, 0)
fourbar = Mechanism(4, 2, (3,4), [bar1; bar2; bar3; bar4])
```

I have no idea why a method error is thrown, since Quantity is a subtype of Number, and also is abstract enough to contain different concrete subtypes of Quantity, shouldn’t the constructor create the datatype?