// c# 7.1
        static bool IsDefault<T>(T value) => Equals(value, default);

        static void Main(string[] args)
        {
            Console.WriteLine(IsDefault(0));
            Console.ReadKey();
        }

static bool IsDefault<T>(T value) => Equals(value, default(T));

Y>>static bool IsDefault<T>(T value) => Equals(value, default(T));
Y>>

You can now omit the type on the right-hand side of the initialization:
C#

Func<string, bool> whereClause = default;

static bool IsDefault<T>(T value){
 T d = default(T);
 return Equals(value, d); // Работает как надо
}

        IL_0000: ldarg.0
        IL_0001: box !!T
        IL_0006: ldnull
        IL_0007: call bool [mscorlib]System.Object::Equals(object, object)

        // Method begins at RVA 0x2050
        // Code size 26 (0x1a)
        .maxstack 2
        .locals init (
            [0] !!T
        )

        IL_0000: ldarg.0
        IL_0001: box !!T
        IL_0006: ldloca.s 0
        IL_0008: initobj !!T
        IL_000e: ldloc.0
        IL_000f: box !!T
        IL_0014: call bool [mscorlib]System.Object::Equals(object, object)

static bool IsDefault<T>(T value) => EqualityComparer<T>.Default.Equals(value, default);

int? a = null;
int? b = 5;
if (a <= b == Comparer<int?>.Default.Compare(a, b) <= 0) // что не так-то?
  ...