Cannot implicitly open existentials in a tuple element

Using Swift
1

given a type like

public
struct IntegerOverflowError:Error, Sendable 
{
    public
    let number:Number
    
    public
    let overflows:any FixedWidthInteger.Type
}

why does this work?

extension IntegerOverflowError:Equatable
{
    public static
    func == (lhs:Self, rhs:Self) -> Bool
    {
        Self.eq(lhs: lhs, number: rhs.number, overflows: rhs.overflows)
    }

    private static
    func eq<Integer>(lhs:Self, number:Number,
        overflows _:Integer.Type) -> Bool
        where Integer:FixedWidthInteger
    {
        lhs.number == number && lhs.overflows is Integer.Type
    }

but not this?

    private static
    func eq2<Integer>(lhs:Self,
        rhs:(number:Number, _:Integer.Type)) -> Bool
        where Integer:FixedWidthInteger
    {
        lhs.number == rhs.number && lhs.overflows is Integer.Type
    }
}