Why can't I write Nullable>?

Question

The definition of Nullable is:

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

The constraint

Answer 1

As others have said, the spec prohibits this.

Digging deeper, it's worth realising that you can make your own struct that allows this pattern:

struct Nestable where T : struct { /* ... */ }

new Nestable>(); // This works just fine

The prohibition of nested nullables can not be expressed using the type system available to you and me. It is only enforced by a special case in the compiler (CS0453).

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Aside: The new() constraint shown in the question doesn't actually exist on System.Nullable. new() constraints are prohibited when using the struct constraint.

CS0451: The 'new()' constraint cannot be used with the 'struct' constraint

All structs support default initialisation anyway.