how to overload an assignment operator in swift

That's not possible - as outlined in the documentation:

It is not possible to overload the default assignment operator (=). Only the compound assignment operators can be overloaded. Similarly, the ternary conditional operator (a ? b : c) cannot be overloaded.

If that doesn't convince you, just change the operator to +=:

func +=(left: inout CGFloat, right: Float) {
    left += CGFloat(right)
}

and you'll notice that you will no longer get a compilation error.

The reason for the misleading error message is probably because the compiler is interpreting your attempt to overload as an assignment

You can not override assignment but you can use different operators in your case. For example &= operator.

func &= (inout left: CGFloat, right: Float) {
    left = CGFloat(right)
}

So you could do the following:

var A: CGFLoat = 1
var B: Float = 2
A &= B

By the way operators &+, &-, &* exist in swift. They represent C-style operation without overflow. More

how to overload an assignment operator in swift

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