[nem-en] Nullable types

[Kamil Skalski]

Just like I thought, int? has quite much compiler level changes. First
of all, it is a struct, so simply it cannot be assigned null. Also,
when we take a look at following program:

///////////////////
class B {
  static void Main () {
    int? x = null;
    if (x == null)
      System.Console.WriteLine ("is null");
    System.Console.WriteLine (x.ToString ());
    int? y = 1;
 }
}
/////////////////////////

Then
- yes, it will compile :)
- yes, it will print "is null"
- no, it will not crash with NullReferenceException, it will print an
empty string
- no, IL for this program have nothing to do with IL for any other
type instead of Nullable:

////////////////////////////
    .locals init (valuetype [mscorlib]System.Nullable`1<int32> V_0,
             valuetype [mscorlib]System.Nullable`1<int32> V_1,
             bool V_2)
    IL_0000:  nop
    IL_0001:  ldloca.s   V_0
    IL_0003:  initobj    valuetype [mscorlib]System.Nullable`1<int32>
    IL_0009:  ldloca.s   V_0
    IL_000b:  call       instance bool valuetype
[mscorlib]System.Nullable`1<int32>::get_HasValue()
    IL_0010:  stloc.2
    IL_0011:  ldloc.2
    IL_0012:  brtrue.s   IL_001f

    IL_0014:  ldstr      "is null"
    IL_0019:  call       void [mscorlib]System.Console::WriteLine(string)
    IL_001e:  nop
    IL_001f:  ldloca.s   V_0
    IL_0021:  constrained. valuetype [mscorlib]System.Nullable`1<int32>
    IL_0027:  callvirt   instance string [mscorlib]System.Object::ToString()
    IL_002c:  call       void [mscorlib]System.Console::WriteLine(string)
    IL_0031:  nop
    IL_0032:  ldloca.s   V_1
    IL_0034:  ldc.i4.1
    IL_0035:  call       instance void valuetype
[mscorlib]System.Nullable`1<int32>::.ctor(!0)
///////////////

So there are at least several compiler supported changes:
- treat  'null : Nullable[T]'  as Nullable[T]();
- treat  '(x : Nullable [T]) == null' as  x.HasValue
- treat  'def x : Nullable[int] = 1' as  'def x = Nullable (1);'



--
Kamil Skalski
http://nazgul.omega.pl