class A {}
    class B
    public static @:(_ : B) : A
        A()
    def a = A()
    def b = B()
    // expected B-, got A in computation branch:
    def c = if (a==null) b else a

def p = (10 : int?) :> int; 
// это был пример с наллаблами, но проблема распростроняется и на все другие типы, например:
class Y['t]
  mutable _t : 't
  public this(t : 't)
    _t = t
  public static @:> (y : Y['t]) : 't
    y._t
  
def y = Y.[int](10)
def x = y :> int

tv 't.836 defined in Y and accessed from _N_AutoModule

using Nemerle;
using Nemerle.Compiler;
using Nemerle.Compiler.Parsetree;
using Nemerle.Compiler.Typedtree;
using Nemerle.Text;

namespace Nemerle.Core
{
  macro @?? (exprA, exprB) 
  {
    def refEq = System.Type.ReferenceEquals;
    def ref3Eq = (t1,t2,t3) => refEq(t1, t2) || refEq(t1, t3);
    def tnullable = typeof (System.Nullable[_]);
    def toptNone = typeof (Nemerle.Core.option[_].None);
    def toptSome = typeof (Nemerle.Core.option[_].Some);

    def typer = Macros.ImplicitCTX ();
    def hA = typer.TypeExpr (exprA).Type.Hint;
    def hB = typer.TypeExpr (exprB).Type.Hint;
    def tx = (hA, hB);

    match (tx) {
      | (Some (Class (tiA, _)), Some (Class (tiB, _))) 
        when tiA.IsValueType && refEq (tiA.SystemType, tnullable)
                && !refEq (tiB.SystemType, tnullable)
             || ref3Eq (tiA.SystemType, toptNone, toptSome)
                && !ref3Eq (tiB.SystemType, toptNone, toptSome) =>

          <[ if ($exprA.HasValue) $exprA.Value else $exprB ]>;

      | (Some (Class (tiA, _)), Some (Class (tiB, _))) 
        when tiA.IsValueType && refEq (tiA.SystemType, tnullable)
                && refEq (tiB.SystemType, tnullable) =>

          <[ if ($exprA != null) $exprA else $exprB ]>;

      | (Some (Class (tiA, _)), Some (Class (tiB, _))) 
        when ref3Eq (tiA.SystemType, toptNone, toptSome)
                && ref3Eq (tiB.SystemType, toptNone, toptSome) =>

          <[ if ($exprA.IsSome) $exprA else $exprB ]>;

      | (Some (Class (tiA, _)), Some (Class (_, _))) 
        when tiA.IsValueType =>

          Message.FatalError (exprA.Location, 
                              $"`$tiA' is not a reference or nullable type "
                              "as required by the `??' operator");

      | _ => 
          <[ if ($exprA != null) $exprA else $exprB ]>;
    }
  }
}

using System.Console;



def GetExprType['t] (_ : 't)
{
  typeof ('t)
}



def rn = null;
def r1 = "str1";
def r2 = "str2";

assert (rn ?? r2 == "str2");
assert (GetExprType(rn ?? r2).Equals (typeof (string)));
WriteLine($"$(rn ?? r2)");

assert (r1 ?? r2 == "str1");
assert (GetExprType(r1 ?? r2).Equals (typeof (string)));
WriteLine($"$(r1 ?? r2)");



def dn : double? = null;
def d1 : double? = 0.1;
def d2 : double? = 0.2;

assert ((dn ?? d2).Value == 0.2);
assert (GetExprType(dn ?? d2).Equals (typeof (double?)));
WriteLine($"$(dn ?? d2)");

assert ((d1 ?? d2).Value == 0.1);
assert (GetExprType(d1 ?? d2).Equals (typeof (double?)));
WriteLine($"$(d1 ?? d2)");

assert (dn ?? 0.3 == 0.3);
assert (GetExprType(dn ?? 0.3).Equals (typeof (double)));
WriteLine($"$(dn ?? 0.3)");

assert (d1 ?? 0.3 == 0.1);
assert (GetExprType(d1 ?? 0.3).Equals (typeof (double)));
WriteLine($"$(d1 ?? 0.3)");



def on = None();
def on2 = None();
def o1 = Some(1);
def o2 = Some(2);

assert ((on ?? o2).IsSome);
assert ((on ?? o2).Value == 2);
assert (GetExprType(on ?? o2).Equals (typeof (option[int])));
WriteLine($"$(on ?? o2)");

assert ((o1 ?? o2).Value == 1);
assert (GetExprType(o1 ?? o2).Equals (typeof (option[int].Some)));
WriteLine($"$(o1 ?? o2)");

assert (on ?? 3 == 3);
assert (GetExprType(on ?? 3).Equals (typeof (int)));
WriteLine($"$(on ?? 3)");

assert (o1 ?? 3 == 1);
assert (GetExprType(o1 ?? 3).Equals (typeof (int)));
WriteLine($"$(o1 ?? 3)");

assert ((on ?? on2).IsNone);
assert (GetExprType(on ?? on2).Equals (typeof (option[int].None)));
WriteLine($"$(on ?? on2)");

ie>    def tnullable = typeof (System.Nullable[_]);
ie>    def toptNone = typeof (Nemerle.Core.option[_].None);
ie>    def toptSome = typeof (Nemerle.Core.option[_].Some);
ie>

          <[ if ($exprA.IsSome) $exprA else $exprB ]>;

          <[ match ($exprA) { | Some(x) => x | _ => $exprB ]>;

assert (GetExprType(on ?? o2).Equals (typeof (option[int])));

assert (GetExprType(on ?? o2).Equals (typeof (int)));

IT>Теперь пора заняться '??='

  macro @??= (exprA, exprB) 
  {
    <[ {$exprA = $exprA ?? $exprB; $exprA} ]>
  }

IT>Теперь пора заняться '??='

def i : int? = 10;
def j = i ?? 2;

mutable a;
def b = a ?? { a = c; a };

int? p = null;
int? q = null;
if (p ?? q == 0); // тут мне 0 нафиг не нужен

IT>mutable a;
IT>def b = a ?? { a = c; a };
IT>

mutable a : int? = null;
def b = a ?? 5; // тут тип int, а если тупо выражать ??= через ??, то получим бредовые ошибки.

Error 1 expected option[int-]-, got int in assigned value: common super type of types [option[int-], int] is just `System.Object', please upcast one of the types to `System.Object' if this is desired E:\Ivan\Projects\MyOwn Projects\Nemerle Projects\Macro\Cns.MacroLibrary.Test\Main.n 78 9 Cns.MacroLibrary.Test
Error 2 there is no member named `HasValue' in System.Object- with type ? E:\Ivan\Projects\MyOwn Projects\Nemerle Projects\Macro\Cns.MacroLibrary.Test\Main.n 78 9 Cns.MacroLibrary.Test
Error 3 there is no member named `Value' in System.Object- with type ? E:\Ivan\Projects\MyOwn Projects\Nemerle Projects\Macro\Cns.MacroLibrary.Test\Main.n 78 9 Cns.MacroLibrary.Test

`5' should have type `option[int]'.

def b = {when (a == null) a = 6; a};