Add optional types

[andrey-zherikov]

I believe optional types should be baked into language, not be a library solution, so:

int? a; // not set by default
int? b=1; // initialized
int* p=null; int? c=*p; // safe to dereference null pointer: value is not set
// etc

[IgorDeepakM]

This is nice and should be used in the library rather than a raw pointers as we have it today. However, it also ties into adding l-value references.

Like this example using AAs.

// Old method

int* p;

p = "hello" in aa;
if (p !is null)
{
    *p = 4;  // update value associated with key
    assert(aa["hello"] == 4);
}

// Newer method

int? p;

p = "hello" in aa;
if (p !is null)
{
    p = 4;  // update value associated with key, p is not updated inside the AA as p is a value
}

// Using references

(ref int)? p;      // What is a nice syntax here? (ref int)?, ref? int    ??????????

p = "hello" in aa;
if (p !is null)
{
    p = 4;  // update value associated with key
    assert(aa["hello"] == 4);
}

[ntrel]

int* p=null;

Null-safety please:

int*? p = null;
int* q = p; // error
p = q; // OK