Add where T : struct? generic type constraint

[AlexRadch]

It is impossible to write one generic method for non-nullable value type and nullable value type.

Many generic methods do not support a nullable value type due to the lack of such a feature.

For example, MemoryMarshal.Сast does not support nullable values

To support MemoryMarshal.Cast method for a nullable value type, you need to create 3 more variants of it with the same code.

namespace System.Runtime.InteropServices;

public static class MemoryMarshal
{
    public static ReadOnlySpan<TTo> Cast<TFrom, TTo>(ReadOnlySpan<TFrom> span) // original from non-nullable to non-nullable only
    where TFrom : struct
    where TTo : struct;

    public static ReadOnlySpan<TTo> Cast<TFrom, TTo>(ReadOnlySpan<TFrom?> span)  // from nullable to non-nullable
    where TFrom : struct
    where TTo : struct;

    public static ReadOnlySpan<TTo?> Cast<TFrom, TTo>(ReadOnlySpan<TFrom> span)  // from non-nullable to nullable
    where TFrom : struct
    where TTo : struct;

    public static ReadOnlySpan<TTo?> Cast<TFrom, TTo>(ReadOnlySpan<TFrom?> span) // from nullable to nullable
    where TFrom : struct
    where TTo : struct;
}

I suggest adding where T : struct? generic type constraint and then such a method with a new constraint will support all 4 options.

    public static ReadOnlySpan<TTo> Cast<TFrom, TTo>(ReadOnlySpan<TFrom> span) // just one
    where TFrom : struct?
    where TTo : struct?;

The interface can only be requested after checking that the value exists:

public static TTo Foo<TFrom, TTo>(TFrom nullableValue)
    where TFrom : struct?, IEquitable<TFrom!> // can be non-nullable value type OR nullable value type
    where TTo : struct? // can be non-nullable value type OR nullable value type
{
    // Before checking, nullableValue does not support IEquitable<TFrom!> constraint
    if (nullableValue is not null value) // value is not nullable here. Always success for a non-nullable value type, a copy is created.
    {
        value.Equals(default); // Here value support IEquitable<TFrom!> constraint
    }
}

If the TFrom is a non-nullable value type, then the checks are always successful, and a copy of the structure is created.