Regression with optional arguments in local function closure

[mmusu3]

Input code

class OptionalArgumentTest
{
    int data;

    void Test(string format)
    {
        TestLocal();

        void TestLocal(int a = 0)
        {
            a.ToString(format);
        }
    }
}

Erroneous output

using System.Runtime.InteropServices;

internal class OptionalArgumentTest
{
	private int data;

	private void Test(string format)
	{
		TestLocal();

		void TestLocal([Optional][DefaultParameterValue(0)] int a)
		{
			a.ToString(format);
		}
	}
}

Details

Tested at commit 587a359
Regressed in #3470

The loop in TypeSystemExtensions.IsDefaultValueAssignmentAllowed does not account for the extra parameter generated for the closure.