[Sergio Pedri (Sergio0694)]

Fixes #1947.

Problem

The AOT source generator's CCW lookup table discovery (GetVtableAttributesToAddOnLookupTable in AotOptimizer.cs) called GetTypeInfo directly on the initializer / assignment / return / argument expressions it inspects. For a conditional (ternary) or switch expression, GetTypeInfo only yields the common target type of all branches (typically object or an interface), and for a cast expression it yields the cast target type. So the concrete types actually being boxed or cast were never discovered and got no CCW vtable entries or generic instantiations, failing at runtime under Native AOT.

// Nothing here was discovered: 'List<int>' and 'List<string>' both got no CCW vtable
object obj = GetFlag() ? new List<int>() : (object)new List<string>();

Fix

Add an AddVtableAttributesForExpression helper that looks through parenthesized, cast, conditional (ternary), and switch expressions to reach the concrete leaf expressions that flow into the target, and route every expression-based call site through it. For casts the target type is still processed directly, so existing (List<int>)value discovery is preserved while the operand ((object)new List<string>()) is now also discovered.

Testing

• Source generator unit tests (src/Tests/SourceGeneratorTest/AotOptimizerTests.cs): run WinRTAotSourceGenerator over ternary, switch, and cast boxing and assert the concrete types are registered in the generated CCW vtable lookup table. The three positive tests fail without the fix and pass with it; a control test guards against discovering types that aren't actually boxed.
• AOT CCW functional test (src/Tests/FunctionalTests/CCW/Program.cs): box List<T> instances only reachable through a ternary or switch expression and validate, under Native AOT, that their CCWs expose the expected IVector<T> vtable via the reported runtime class name.

Verified locally: SourceGeneratorTest (68 tests) and DiagnosticTests (364 tests) pass, and both the Roslyn 4.8 and Roslyn 4.12 generator variants build cleanly.

Grouped as three commits: the generator fix, the AOT functional test, and the unit tests.

[Copilot]

Pull request overview

This PR fixes Native AOT CCW lookup-table discovery in the WinRT AOT source generator by ensuring it can “see through” ternary, switch, parenthesized, and cast expressions to the concrete leaf expressions that actually produce boxed values. This addresses issue #1947 where only the common/converted type (often object or an interface) was observed, causing missing CCW vtable entries and runtime failures under Native AOT.

Changes:

• Updated AotOptimizer.GetVtableAttributesToAddOnLookupTable to route expression-based sites through a new AddVtableAttributesForExpression helper that recursively inspects ternary/switch/cast/parenthesized expressions.
• Added generator unit tests validating discovery for ternary, switch, and cast scenarios (plus a control test to prevent over-eager discovery).
• Added a functional CCW regression test validating List<T> CCWs expose the expected IVector<T> vtable under Native AOT when the lists are only reachable through ternary/switch expressions.

Reviewed changes

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

File	Description
src/Authoring/WinRT.SourceGenerator/AotOptimizer.cs	Adds an expression-walking helper to discover concrete boxed/cast types in ternary/switch/cast/parenthesized expressions for CCW vtable lookup generation.
src/Tests/SourceGeneratorTest/AotOptimizerTests.cs	Adds source generator regression tests asserting concrete generic types are discovered in generated CCW lookup table sources.
src/Tests/FunctionalTests/CCW/Program.cs	Adds Native AOT functional regression coverage ensuring CCWs created from values boxed via ternary/switch expose expected IVector<T> runtime class names.

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