Why can't I define a default constructor for a struct in .NET?

Question

In .NET, a value type (C# struct) can\'t have a constructor with no parameters. According to this post this is mandated by the CLI specification. What happens i

Answer 1

public struct Rational 
{
    private long numerator;
    private long denominator;

    public Rational(long num = 0, long denom = 1)   // This is allowed!!!
    {
        numerator   = num;
        denominator = denom;
    }
}