C#类型转换错误,尽管通用约束

abstract class A { }

static class S
{
    public static void DoFirst(A argument) { }
    public static void DoSecond(ICollection<A> argument) { }
}

static class P<T>
    where T : A,new()
{
    static void Do()
    {
        s.DoFirst(new T());             // this call is OK

        s.DoSecond(new List<T>());      // this call won't compile with:

        /* cAnnot convert from 'System.Collections.Generic.List<T>'
           to 'System.Collections.Generic.ICollection<A>' */
    }
}

通用约束不应该确保List< T>.确实是ICollection< A>？