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To produce optional parameters for a method from an interface reflected in Roslyn, you can use the Optional attribute from the System.Runtime.InteropServices namespace. Here's an example code snippet:

using System.Runtime.InteropServices;

interface IMyInterface
{
    void MyMethod(int x, [Optional] int y);
}

class MyClass : IMyInterface
{
    public void MyMethod(int x, [Optional] int y)
    {
        // method implementation
    }
}

In this example, we have an interface IMyInterface with a method MyMethod. The second parameter y is marked as optional using the [Optional] attribute.

When implementing the interface in a concrete class MyClass, we can omit the y parameter if we don't need it. This allows callers of the method to provide only the necessary parameters without having to specify a value for y if it's not needed.

Note that in order to use the [Optional] attribute, you need to ensure that the System.Runtime.InteropServices namespace is included in your project.