[jeremydmiller]

Part of #5649 — deliberately not Fixes, see below.

Two hazards in how DCB tag types reach the store, both invisible until Native AOT. Neither needs an API decision, so they're split out from the RegisterTagType question still under discussion on the issue.

Build<TDoc>() closed a generic it already knew

EventGraph.Build<TDoc>() went through CloseAndBuildAs for a [BoundaryAggregate] even though both type arguments are known where it stands — TDoc from the caller, and string unconditionally per #324. ILC has no such instantiation, so every DCB boundary fetch threw MissingMethodException. Writing it out roots the instantiation at compile time.

It carries a CS8714, the same one ProjectionOptions.cs:374 already takes for the identical construction: Build<TDoc>() is public and unconstrained, so the compiler can't see that TDoc satisfies notnull. Constraining a public signature to silence it would be source-breaking for no runtime gain — CloseAndBuildAs never checked it either.

Auto-discovery dropped tag types in silence

autoDiscoverTagTypesFromProjections swallowed every exception through one bare catch. An identity that simply isn't a strong-typed identifier should be passed over without a word — but the AOT failure of MakeGenericMethod was being passed over just as quietly, leaving a store running with the tag type absent and nothing anywhere saying so. The queries that need it then fail far from the cause. @skoging called this one from reading the code; it's real.

It is still not rethrown. This runs inside the store's Initialize, and a throw would stop an application from starting over a feature it may not use — the same trade the CreateTagTypeMethod lookup immediately above it already makes, and the lesson from #5619. It logs a warning naming the identity type, the projection, and the remedy.

Verified by running the native binary

Not by a clean publish, which proves nothing here. Marten.AotRuntimeSmoke gains a boundary-aggregate check. On the unfixed code, published natively for osx-arm64 and run, it reproduces the issue's reported exception verbatim:

FAIL boundary aggregate aggregator source
System.MissingMethodException: No parameterless constructor defined for type
'Marten.Events.Aggregation.SingleStreamProjection`2[Toegangsgrens,System.String]'.
   at JasperFx.Core.Reflection.TypeExtensions.CloseAndBuildAs[T](Type, Type[])
   at Program.<>c__DisplayClass0_0.<<Main>$>b__40()
Marten AOT runtime smoke FAILED — 1 read path(s) broken under Native AOT.

With the fix: OK boundary aggregate aggregator source, 24 of 24, exit 0.

The check calls Build<TDoc>() directly rather than going through AggregateByTagsAsync, because reaching the boundary fetch needs a registered tag type and RegisterTagType<TTag> closes its own open generic — the separate hole this PR doesn't close. This isolates the one site the fix moves, and it's the site the reported exception came from.

Testing

• Marten.AotRuntimeSmoke, native osx-arm64: 24/24, red-checked as above.
• DCB suite (*Dcb*): 196 passed, 2 skipped, 0 failed.
• EventSourcingTests: 2474 passed, 27 skipped, 0 failed.

Two xUnit tests are added. Be aware of what they are and aren't: the Build<TDoc>() one is a characterization test — it pins the constructed type so the refactor can't change behaviour, and it passes on master too, because the AOT failure can't be reproduced under the JIT. The native check above is the real red/green. The second test pins the regression the logging change risks, which is warning ordinary applications on every start about identities there was never anything to discover from.

Why not Fixes #5649

RegisterTagType<TTag> still closes TagTypeRegistration<,> with MakeGenericType, and Create<TTag> additionally reflects over the tag type through ValueTypeInfo.ForType. That needs a reflection-free construction path in JasperFx.Events plus a new Marten overload — discussed in this comment, including why a source generator is the right fourth step rather than the first.

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https://claude.ai/code/session_01481eiEZg1Bv6pu4DrgPRg3