Why can't I write Nullable>?

Question

The definition of Nullable is:

[SerializableAttribute]
public struct Nullable where T : struct, new()

The constraint

Answer 1

Because it's in the C# spec (section 4.4.4):

If the constraint is the value type constraint (struct), the type A must satisfy one of the following:

• A is a struct type or enum type, but not a nullable type. Note that System.ValueType and System.Enum are reference types that do not satisfy this constraint.
• A is a type parameter having the value type constraint (§10.1.5).