InvalidProgramException when using a conditional return null with hyperbee.expressions

[victorr99]

Hi!

I'm trying to compile an expression which uses the BlockAsync and Await methods of hyperbee.expressions. The following code produces an InvalidProgramException when being run:

Task<object> Something()
{
    return Task.FromResult<object>(new object());
}

var dlgarg = Parameter(typeof(Func<Task<object>>), "dlg");
var resvar = Variable(typeof(object), "res");
var retlbl = Label(typeof(object), "return");

var lambda = Lambda<Func<Func<Task<object>>, Task<object>>>(
    BlockAsync(
        [resvar],
        Assign(
            resvar,
            Await(
                Invoke(dlgarg))),
        IfThen(
            Equal(resvar, Constant(null, typeof(object))),
            Return(retlbl, Constant(null, typeof(object)))),
        Return(retlbl, resvar, typeof(object)),
        Label(
            retlbl,
            Constant(null, typeof(object)))),
    dlgarg);

var fn = lambda.CompileFast();
Console.WriteLine(await fn(Something));

The exception being raised:

Unhandled exception. System.InvalidProgramException: Common Language Runtime detected an invalid program.
   at (ArrayClosure, StateMachine1)
   at FastExpressionCompiler.ExpressionCompiler.CurryClosureActions.<>c__DisplayClass2_0`2.<Curry>b__0(T1 t1)
   at StateMachine1.IAsyncStateMachine.MoveNext()
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.Start[TStateMachine](TStateMachine& stateMachine)
   at (ArrayClosure, Func`1)
   at Program.Main() in C:\something\Program.cs:line 51
   at Program.<Main>()

The expression tree does compile correctly with the .NETs Compile() method. If you remove the IfThen from the expression tree, like stated in the comment in the example code, it is also working like expected.
The code also works properly when the BlockAsync and Await methods are removed. Since it is working properly with .NET's Compile() method, I assume this is a bug in FEC?

I'm using version 1.3.2 of hyperbee.expressions.

Edit 2026-02-02:
I've tried to narrow the issue down in such a way that it doesn't involve hyperbee.extensions, but I haven't had success so far. I'll try again at a later time.