Fix source generator for void-returning activity functions

[Copilot]

The source generator produced invalid C# code for activity functions returning void or non-generic Task, attempting to generate Task<void> which doesn't compile.

Example of the issue:

[Function(nameof(FlakeyActivity))]
public static void FlakeyActivity([ActivityTrigger] object _)
{
    throw new ApplicationException("Kah-BOOOOM!!!");
}

// Generator produced invalid code:
// public static Task<void> CallFlakeyActivityAsync(...)  // ❌ doesn't compile

Changes

• DurableFunction.cs: Added ReturnsVoid flag detected via SpecialType.System_Void and non-generic Task comparison using SymbolEqualityComparer
• DurableTaskSourceGenerator.cs: Generate Task (not Task<T>) for void activities, calling CallActivityAsync without generic type parameter
• TypedParameter.cs: Make object parameters nullable via SpecialType check while preserving source nullability for other types

Generated code after fix:

// Void return → Task (no generic)
public static Task CallFlakeyActivityAsync(this TaskOrchestrationContext ctx, object? _, TaskOptions? options = null)
{
    return ctx.CallActivityAsync("FlakeyActivity", _, options);
}

// Task<T> return → Task<T> (unchanged)
public static Task<string> CallSayHelloAsync(this TaskOrchestrationContext ctx, string name, TaskOptions? options = null)
{
    return ctx.CallActivityAsync<string>("SayHello", name, options);
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Source generator doesn't handle activity functions that return void</issue_title>
<issue_description>I have the following activity definition:

[Function(nameof(FlakeyActivity))]
public static void FlakeyActivity([ActivityTrigger] object _)
{
    throw new ApplicationException("Kah-BOOOOM!!!");
}

Source-gen tried to produce the following:

public static Task<void> CallFlakeyActivityAsync(this TaskOrchestrationContext ctx, object _, TaskOptions? options = null)
{
    return ctx.CallActivityAsync<void>("FlakeyActivity", _, options);
}

Obviously, this didn't compile. It seems we need to handle this as a special-case.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Source generator doesn't handle activity functions that return void #119

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