Fix CS8603 error for nullable reference types in SourceGen binding paths

[Copilot]

Fix Implemented and Tested - Review Feedback Addressed

Issue Analysis:

• When binding to source.SomeNullableObject.SomeStringProperty where SomeNullableObject is nullable and SomeStringProperty is a reference type, the generated code produces CS8603 error
• The bug was in CompiledBindingMarkup.cs in the TryParsePath method (lines 410-422)
• Lines 410-420 applied nullable annotation, but line 422 immediately overwrote it with previousPartType

Fix Applied:

• Moved the nullable annotation logic AFTER the propertyType = previousPartType assignment
• Only apply nullable annotation for reference types (not value types, as they use ?? default fallback)
• This ensures the TypedBinding signature correctly reflects nullable reference types in the binding path

Review Feedback Addressed:

• Changed StringProperty from string? to string in test cases (nullability comes from nullable object, not the property)
• Removed WriteLine debugging statements
• Converted first test to snapshot test with full expected output
• Split combined assertions into separate asserts
• Removed unnecessary blank lines from expected output

Testing:

• Added test cases for nullable reference types
• Fixed the bug in CompiledBindingMarkup.cs
• All SourceGen.UnitTests pass (4/4 CompiledBindings tests)
• All BindingSourceGen.UnitTests pass (113/113 tests)
• Ready for final review

Files modified:

• src/Controls/src/SourceGen/CompiledBindingMarkup.cs - Fixed nullable annotation logic
• src/Controls/tests/BindingSourceGen.UnitTests/IntegrationTests.cs - Updated test cases per review feedback
• src/Controls/tests/SourceGen.UnitTests/InitializeComponent/CompiledBindings.cs - Updated test cases per review feedback

Original prompt

---

This section details on the original issue you should resolve

<issue_title>error CS8603: Possible null reference return in generated code using SourceGen</issue_title>
<issue_description>### Description

cc @simonrozsival, c.f. discussion in MauiVerse Discord

Given the following xaml:

<Entry Text="{Binding SomeNullableObject.SomeStringProperty}"/>

using <MauiXamlInflator>SourceGen</MauiXamlInflator>
this will cause a

error CS8603: Possible null reference return.

in the generated delegate

return Create(
	getter: static source => source.SomeNullableObject?.SomeStringProperty,
	extension.Mode,
	extension.Converter,
	extension.ConverterParameter,
	extension.StringFormat,
	extension.Source,
	extension.FallbackValue,
	extension.TargetNullValue);

I guess this is related to #32402, where something was implemented for value types, not reference types.

I guess it should work for strings specifically, as we often want to bind nullable strings to Entry, Editor, or other wrapped custom controls.

Steps to Reproduce

No response

Link to public reproduction project repository

No response

Version with bug

10.0.0-rc.2

Is this a regression from previous behavior?

No, this is something new

Last version that worked well

No response

Affected platforms

Android, Windows, iOS

Affected platform versions

No response

Did you find any workaround?

No response

Relevant log output

```</issue_description>

<agent_instructions>I believe this fix will be very similar to https://github.com/dotnet/maui/pull/32402. Ideally reuse or generalise some of that code to reduce code complexity.</agent_instructions>

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

<comments>
</comments>

• Fixes error CS8603: Possible null reference return in generated code using SourceGen #32478

---

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[simonrozsival]

@copilot PTAL

[simonrozsival]

/azp run

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[Copilot]

Pull Request Overview

This PR fixes CS8603 errors in SourceGen-generated code when binding to nullable reference types through a nullable path (e.g., source.SomeNullableObject?.SomeStringProperty).

Key Changes:

• Fixed bug in CompiledBindingMarkup.cs where nullable annotation was being overwritten
• Added comprehensive test coverage for nullable reference types in binding paths
• Tests verify both the generated code structure and absence of CS8603 errors

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/Controls/src/SourceGen/CompiledBindingMarkup.cs	Fixed nullable annotation logic by moving assignment before annotation
src/Controls/tests/SourceGen.UnitTests/InitializeComponent/CompiledBindings.cs	Added snapshot test and error verification test for nullable reference types
src/Controls/tests/BindingSourceGen.UnitTests/IntegrationTests.cs	Added integration tests for SetBinding and Create with nullable paths

[ceubanks14]

Is this part of the latest .NET release today?