Summary

Fixes #730.

Microsoft.DurableTask.Generators produced code that does not compile when a class-based activity uses a nullable input type such as object?. For example:

[DurableTask]
public class WorkerCapacityReviewActivity : TaskActivity<object?, WorkerCapacityReviewResult> { ... }

generated:

public static async Task<WorkerCapacityReviewResult> WorkerCapacityReviewActivity(
    [ActivityTrigger] object? input = default, string instanceId, FunctionContext executionContext)

The = default makes input optional, but it is followed by the required instanceId and executionContext parameters — an optional parameter cannot precede required ones (CS1737), so the generated file fails to compile.

Root cause

AddActivityFunctionDeclaration in DurableTaskSourceGenerator.cs appended = default to the [ActivityTrigger] parameter whenever the input type ended in ?. Unlike the *CallAsync helpers, the trigger parameter is followed by required parameters, so a default value is both invalid and unnecessary (the Functions runtime always binds the input).

Fix

• Remove the = default append in AddActivityFunctionDeclaration only, with a comment explaining why this method differs from the *CallAsync helpers.
• The three other = default sites (ScheduleNew…InstanceAsync, Call…Async sub-orchestrator/activity helpers) are intentionally left unchanged: their input is followed by an optional options parameter, so input = default, options = null is valid C#.

Tests

• Extended the Activities_ClassBasedSyntax theory with an object? input case (added #nullable enable to the input snippet). The expected output verifies the asymmetry: the CallMyActivityAsync helper keeps object? input = default (valid — followed by optional options), while the [Function] trigger emits object? input (no default).
• Verified the new case fails before the fix (reproduces CS1737 / signature mismatch) and passes after.
• Full Generators.Tests suite: 89/89 passing.