Map derived instances and array elements in generated code as the engine does

[arnelirobles]

Generated code planned every nested member, collection element and extension receiver from its declared type. A derived instance was mapped as the base, where the engine maps by the runtime type.

What changed in the emitter:

• A nested helper whose source type can be derived from (a class that is not sealed, or an interface) checks source.GetType() and hands anything that is not exactly the declared type to Mapper.MapTo<TDest>((object)source). A sealed type or a struct gets no check.
• The generated MapTo extension does the same for the declared source, so Animal a = new Dog(); a.MapTo<PetDto>() no longer drops Breed. This fourth case is not in the issue. It showed up while writing the rows.
• An array into a List<T> of the same class maps each element through a nested helper instead of passing the instance through. If that element class cannot be generated, the pair is refused with MSG001.

Proof:

• Seven new rows in LaneConformanceTests. Four fail on main (nested member, list and array elements, Tag[] into List<Tag>, derived instance through the extension). The other three are controls: the declared type itself, a null member, and a cycle that only a derived type adds, which ends at the same depth in both lanes.
• The five new pairs are in the list TheTableCoversEveryPairTheAssemblyDeclares checks, and EveryDeclaredPairWasActuallyGenerated passes, so the rows compare generated code against the engine and not the engine against itself.
• dotnet test Mapsicle.sln -c Release: all green on net8.0 and net10.0 (SourceGen 95, core 532, allocation budgets 10).
• --band on an M1, benchmark types not sealed so the check is in the measured path: 0.94 before, 0.97 and 0.97 after, 792 B on both sides each time. The before run was noisy (hand written at plus or minus 6 ns).
• NativeAOT smoke published and run locally for osx-arm64: all passed.

One behaviour to know about: under NativeAOT a derived instance whose own pair is not declared now throws NotSupportedException from the engine, where it used to come back with the base members only. The README says to declare the derived pair.

Review ran in the same session as the change, so finder and judge shared a context.

After review: a collection whose destination element is an interface (IThing[] or Thing[] into List<IThing>) is now refused with MSG001. Generated code put the source instances in the list and the engine leaves each element null. InterfaceElementPairsAreRefusedAndMapAsTheEngineDoes failed before the change. No emitted code changes for any pair that is still generated, so the band numbers above stand.

Closes #73

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📥 Commits

Reviewing files that changed from the base of the PR and between ae6c8c1 and 3ac8cc6.

📒 Files selected for processing (4)

• README.md
• changelog.d/sourcegen-runtime-types.fixed.md
• src/Mapsicle.SourceGen/MapperGenerator.cs
• tests/Mapsicle.SourceGen.Tests/LaneConformanceTests.cs

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[Copilot]

Copilot review overview

🟡 Changes recommended

Interface-typed members and collection elements still violate the documented runtime-lane behavior.

Review effort: Balanced
Findings: 1

Open (1)

• Reject interface element destinations before identity assignment · New

What changed in this PR

Aligns source-generated mappings with runtime behavior for derived instances and array elements.

Changes:

• Dispatches derived source instances to the runtime mapper.
• Copies class elements when converting arrays to lists.
• Adds conformance tests and documents NativeAOT behavior.

File	Description
src/​Mapsicle.SourceGen/​MapperGenerator.cs	Adds runtime-type guards and element mapping.
tests/​Mapsicle.SourceGen.Tests/​LaneConformanceTests.cs	Adds polymorphism and aliasing coverage.
README.md	Documents derived-type fallback behavior.
changelog.d/​sourcegen-runtime-types.fixed.md	Records the fixes and NativeAOT impact.

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