C# 7.3 Enum constraint: Why can't I use the nullable enum?

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Now that we have enum constraint, why doesn't compiler allow me to write this code?

public static TResult? ToEnum<TResult>(this String value, TResult? defaultValue)     where TResult : Enum {     return String.IsNullOrEmpty(value) ? defaultValue : (TResult?)Enum.Parse(typeof(TResult), value); } 

The compiler says:

Error CS0453 The type 'TResult' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'Nullable'

You can, but you have to add another constraint: the struct constraint.

public static void DoSomething<T>(T? defaultValue) where T : struct, Enum { } 


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