fixed generation of Sum<X, N0>, N0 is prohibited in Sum

src/INumeral.cs

@@ -4,8 +4,7 @@ public interface INumeral {
         int Num { get; }
     }
 
-    public interface INumeral<T> {
-        int Num { get; }
+    public interface INumeral<T>: INumeral {
     }
 
     public static class Numeral<T> where T : unmanaged {
@@ -18,7 +17,9 @@ public static Type GetNType(int n) {
             while (n >= pwr) {
                 if ((n & pwr) == pwr) {
                     var powerOfTwo = PowersOfTwo<T>.Get(pwr);
-                    result = typeof(Sum<,,>).MakeGenericType(typeof(T), powerOfTwo, result);
+                    result = result == typeof(N0<T>)
+                        ? powerOfTwo
+                        : typeof(Sum<,,>).MakeGenericType(typeof(T), powerOfTwo, result);
                 }
 
                 pwr <<= 1;

src/Sum.cs

@@ -16,6 +16,9 @@ static Sum()
             if (n1 <= n2)
                 throw new ArgumentException("Consistency: T1 must always be > T2. Swap them."
                     + $"\n{typeof(Sum<T1, T2>).Name}: {typeof(T1).Name}={n1},{typeof(T2).Name}={n2}");
+            if (n1 == 0 || n2 == 0)
+                throw new ArgumentException("Consistency: T1 and T2 must always be > 0."
+                    + $"\n{typeof(Sum<T1, T2>).Name}: {typeof(T1).Name}={n1},{typeof(T2).Name}={n2}");
             var t1 = typeof(T1);
             var t2 = typeof(T2);
             if (t2.IsConstructedGenericType && t2.GetGenericTypeDefinition() == typeof(Sum<,>))
@@ -41,5 +44,9 @@ public int Num {
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             get => this.Item1.Num + this.Item2.Num;
         }
+
+        static Sum() {
+            default(Sum<T1, T2>).Num.GetHashCode();
+        }
     }
 }

tests/NumeralTests.cs

@@ -0,0 +1,17 @@
+namespace TypeNum {
+    using System;
+
+    using Xunit;
+    public class NumeralTests {
+        [Fact]
+        public void GetNumeralType() {
+            var n5int = Numeral<int>.GetNType(5);
+            Assert.Equal(typeof(Sum<int, N4<int>, N1<int>>), n5int);
+        }
+
+        [Fact]
+        public void Sum0_Prohibited() {
+            Assert.Throws<TypeInitializationException>(() => default(Sum<int, N2<int>, N0<int>>).Num.GetHashCode());
+        }
+    }
+}