How to check if variable of some type can be of null value?

[Adam D Ruppe]

On Saturday, 24 July 2021 at 18:10:07 UTC, Alexey wrote:
> The goal I with to achieve by this check - is to use template 
> and to assign value to variable basing on it's ability to 
> accept null as a value.

The most direct representation of that is __traits(compiles, (T 
t) { t = null; });


Another cool trick to consider is:

is(typeof(null) : Whatever)


What that means is if the null literal will implicitly convert to 
Whatever type. This means you can pass `null` as an argument to a 
function accepting Whatever. Thus it includes pointers, classes, 
interfaces, arrays. But does NOT include structs, even if they 
have a null accepting constructor / opAssign since you still must 
explicitly construct them.

struct S {
    void opAssign(typeof(null) n) {}
}

void main() {
   S s;
   s = null; // allowed due to opAssign
}

pragma(msg, is(typeof(null) : S)); // FALSE because this check 
only looks for implicit conversion, not user-defined assign 
overloads or constructors.

The traits compiles check will allow this, since it is looking at 
assign... but the traits compiles will say false if it gets a 
`const` type since obviously then assign is not allowed, even if 
implicit conversion would be.

Depending on your needs you might use one of these, or perhaps 
both.