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[PLAN-0046] Effective Interface Contract for Inherited Static Abstract Members

Status: Done

Implements: ADR-0046

Goal

A test-double-eligible interface whose closure declares a static abstract member (method, property, or operator) that's already resolved by a more-derived interface in the same closure (C#'s own "most specific implementation" rule) generates and resolves through UseGeneratedTestDoubles() alone, with every member — including the previously-blocking one — behaving exactly as the real interface actually behaves. Closes Gate-B (the "can lightsaber-skill remove Compono.NSubstitute entirely" acceptance criterion) end to end, verified by a real second migration pass against IAmazonS3 specifically, not just generator-level coverage.

Scope

Implements ADR-0046's chosen option (effective-interface-contract analysis) in full, per its Decision Outcome. A static abstract member with no override anywhere in its interface's closure stays whole-interface- rejected under the existing CMP0021, unchanged — per ADR-0046, C# itself (CS8920) makes such an interface uncomposable through Compono's generic Resolve<TValue>() regardless of what this generator does, so no stub or fallback behavior is implemented for that case. Out of scope entirely, unchanged by this plan: events, indexers, and variable-argument methods stay whole-interface-rejected under CMP0021 — this plan narrows one specific trigger condition of CMP0021, it doesn't touch its handling of those other three shapes. Per ADR-0042 Amendment 2, a future evidenced need for configurable static-member behavior, or for composing a genuinely unresolved static abstract member some other way, would be its own new roadmap candidate, not a reason to widen this plan's scope now.

No new public API surface: no new diagnostic code, no new exception type, no new emission branch. This plan's earlier draft (still visible in ADR-0046's "Decision Outcome" section, which records it in full rather than silently rewriting) proposed exactly those three things (CMP0033, TestDoubleUnsupportedMemberException, a conformance-only stub emission branch) before implementation-time compile spikes proved the design wrong (would have silently broken IAmazonS3's own real implementation) and unreachable (blocked upstream by CS8920 for the case it would have legitimately applied to) at the same time. All of that machinery is removed, not shipped.

One implementation phase, one PR for the generator fix, its tests, and its docs — the closing lightsaber-skill dogfood is a separate, follow-up PR (see "Notes"): it requires publishing a new Compono preview package with this fix included before lightsaber-skill's own dependency bump can consume it, which can't happen inside this PR.

Tasks

Analyzer: effective-interface-contract resolution

  • src/Compono.Generators/Discovery/TestDoubleAnalyzer.cs: the IMethodSymbol { IsStatic: true, IsAbstract: true, ... } branch and the IPropertySymbol { IsStatic: true, IsAbstract: true } branch (previously unconditional return Failure(...)) each gain one guard, checked first: if interfaceType.FindImplementationForInterfaceMember(member) returns non-null, the member is already resolved by a more-derived interface in the closure (verified against Roslyn directly, not assumed) — continue past it, the same disposition an ordinary non-abstract static member already gets. If it returns null, fall through to the original, unchanged Failure(...) (whole-interface CMP0021 rejection) — a genuinely unresolved static abstract member's disposition doesn't change.
  • Confirm CMP0021's own message/condition is otherwise unchanged for the shapes it still legitimately rejects: events, indexers, variable-argument methods, and a genuinely unresolved static abstract member.
  • src/Compono.Generators/AnalyzerReleases.Unshipped.md: update CMP0021's row description to reflect the narrowed trigger condition (a resolved-via-derived-interface static abstract member no longer fires it at all).

Test coverage

  • test/Compono.Generators.Tests/: generator-level compile-and-verify tests (not diagnostics-only — GeneratorTestHelpers.Verify, which actually reparses and compiles the generated output, catching the class of bug a diagnostics-only assertion would have missed) for the general Roslyn/interface-inheritance rule, covering a static abstract method, property, and operator each resolved by a more-derived interface — confirm no diagnostic fires, the double generates completely normally, and a sibling instance member's Configure()/Verify() surface is unaffected. Separately, confirm a genuinely unresolved static abstract method/property/operator still reports CMP0021 and rejects the whole interface, unchanged from before this ADR.
  • test/Compono.TestDoubles.SampleTests/: a packaged-consumer test (StaticAbstractMemberTests.cs) proving the fix through the real Compono → Compono.Generators → Compono.TestDoubles dependency chain — an IAmazonS3-shaped interface (a base interface declaring a static abstract member, a derived interface re-implementing it) resolves and its ordinary instance member works through Configure()/TestDoubleNotConfiguredException exactly as any other configuration-required member would.
  • test/Compono.TestDoubles.AotSmokeTest/Program.cs: extended with the same IAmazonS3-shaped probe interface pair, proving the resolved-via-derived-interface member doesn't reject the leaf interface under Native AOT/trimming either — verified via a real dotnet publish -p:PublishAot=true + run, not just a JIT test.

Docs/skill alignment

  • docs/packages/compono-testdoubles.md: new "Static abstract members inherited from a base interface" section (near the "Configuration-required members" section) documenting the supported shape with the real IAmazonS3/IAmazonService example; "What it deliberately doesn't do" corrected to say a genuinely unimplemented static abstract member still rejects the interface, not "a static abstract member" unconditionally.
  • docs/reference/diagnostics.md, skills/compono/references/diagnostics.md: CMP0021's entry corrected to describe the narrowed trigger condition and link the new supported-shape documentation.
  • skills/compono/references/testdoubles.md: "still unsupported" prose corrected the same way, plus a short note on the resolved-via-derived-interface exception.
  • skills/compono/SKILL.md, docs/troubleshooting/common-errors.md: checked — neither names static abstract members specifically, no change needed.
  • No docs/reference/api/Compono/ regeneration needed — no new public type was added (the originally-planned TestDoubleUnsupportedMemberException was withdrawn, not shipped).

Gate-B closing dogfood: lightsaber-skill

  • Published Compono/Compono.TestDoubles/Compono.XunitV3 0.5.0-preview.74, including the merged fix (compono#99).
  • Re-ran lightsaber-skill's build/deps-bump-compono-preview-73 branch against the new package. Migrated LightsaberHandlerTests.cs's remaining IAmazonS3 usage (~9 call sites: Substitute.For<IAmazonS3>(), Arg.Any/.Returns() on ListObjectsV2Async) to Compono.TestDoubles' Configure()/ Composer.Create(...) pattern, matching the other four files. ListObjectsV2Async verified (reflection against the real AWSSDK.S3 package) to be non-overloaded, so no discriminator was needed.
  • Removed Compono.NSubstitute from Directory.Packages.props and Lightsaber.Skill.Tests.csproj's PackageReferences entirely, and UseNSubstitute() from GeneratedTestDoublesProfile. Confirmed NSubstitute itself is no longer pulled in even transitively (dotnet list package --include-transitive) — Gate-B's exact wording is "no longer required transitively," not just "no direct reference," and this confirms it.
  • Full 77-test suite passes, verified by running the built Microsoft.Testing.Platform executable directly (not just a clean compile) — 77/77, 0 skipped.
  • AOT scope question resolved with the user directly: lightsaber-skill doesn't need its own new AOT infrastructure — Compono.TestDoubles.AotSmokeTest (already covered by this plan's earlier task section) is sufficient.
  • Recorded the result as RESEARCH-0006 — full success, every bullet above passed, no partial result to report this time.
  • Updated docs/roadmap/post-mvp.md's entry for this candidate: removed from "outstanding" — Gate-B met in full, real Compono.NSubstitute removal, not just fewer call sites.

Critical Files

  • src/Compono.Generators/Discovery/TestDoubleAnalyzer.cs — the two static-abstract branches gain the effective-interface-contract guard.
  • src/Compono.Generators/AnalyzerReleases.Unshipped.md — CMP0021 row description updated.
  • test/Compono.Generators.Tests/TestDoubleVerifyTests.cs, test/Compono.TestDoubles.SampleTests/StaticAbstractMemberTests.cs, test/Compono.TestDoubles.AotSmokeTest/Program.cs — new test coverage.
  • docs/packages/compono-testdoubles.md, docs/reference/diagnostics.md, skills/compono/references/diagnostics.md, skills/compono/references/testdoubles.md — doc/skill alignment.
  • docs/adr/0046-static-abstract-member-conformance-only-generation.md — rewritten in place (still Proposed when the rewrite happened, so no Amendment was needed) to record both the corrected design and the two compile-spike findings that invalidated the original one.
  • docs/roadmap/post-mvp.md, docs/research/0006-lightsaber-skill-testdoubles-gate-b-closing-dogfood.md — the Gate-B closing result.
  • (External repo) ncipollina/lightsaber-skill's Directory.Packages.props, Lightsaber.Skill.Tests.csproj, Composition/GeneratedTestDoublesProfile.cs, Handlers/LightsaberHandlerTests.cs — the actual migration (lightsaber-skill#108).

Test Plan

Matches references/testing.md's existing pattern for this feature area: generator-level compile-and-verify tests for the analyzer fix (not diagnostics-only — full reparse-and-recompile of the generated output), packaged-consumer behavior tests through the real dependency chain, and Native AOT proof. Before this PR merges, it must demonstrate all of the following, not a subset:

  • A static abstract method, property, and operator, each already resolved by a more-derived interface in the closure, no longer reject their declaring interface (CMP0021 doesn't fire; no other diagnostic fires either — the fix doesn't add one).
  • The resolved member's real, existing implementation is preserved exactly — the double doesn't emit anything for it, so there's nothing that could shadow or change its behavior.
  • A genuinely unresolved static abstract method/property/operator (no override anywhere in the closure) still reports CMP0021 and rejects the whole interface, unchanged from before this ADR.
  • Every other, already-supported instance member on the same interface is completely unaffected — same generated behavior as before this plan.
  • Native AOT/trimming still passes (Compono.TestDoubles.AotSmokeTest), including the new resolved-via-derived-interface probe.
  • No new public API surface exists — no new diagnostic code, no new exception type, verified by grep/review, not just by omission.
  • docs/reference/diagnostics.md, skills/compono/references/diagnostics.md, docs/packages/compono-testdoubles.md, and any other skill/doc surface naming static abstract members are current, not stale.

The remaining Gate-B bullets (real IAmazonS3 removal of Compono.NSubstitute in lightsaber-skill, the full 77-test suite, the closing research doc, the roadmap update) landed in the follow-up PR described in "Notes" — all passed, recorded in RESEARCH-0006.

Notes

2026-08-18 — design corrected during implementation, not just implemented. This plan's original draft (ADR-0046's first accepted design) specified conformance-only stub generation: a new CMP0033 diagnostic, a new TestDoubleUnsupportedMemberException type, and a new emitter/template branch generating a throwing explicit static interface implementation. Implementation proceeded far enough to generate real code and run it against real compilation before two compile spikes (both recorded in full in ADR-0046) proved the design wrong: it would have silently shadowed and broken IAmazonS3's own real, working implementation (a more-derived type's own explicit static interface implementation wins over an already-resolved base-interface override — verified by executing the shape, not just reading the spec), and separately, the case it would have legitimately applied to (a genuinely unresolved static abstract member) turned out to be permanently unreachable through Compono's own composition mechanism regardless (CS8920). All three pieces of that original design were removed before this PR, replaced by the effective-interface-contract fix this plan now describes. This is why every "Tasks" bullet above already shows [x] — this plan is being finalized alongside implementation, not written ahead of it, per this repo's convention that a plan is a living document, correctable via Notes rather than treated as a wrong-but-frozen spec.

Dogfood deferred to a follow-up PR, then completed. The generator-side fix (compono#99) merged and shipped as Compono 0.5.0-preview.74 independently of the lightsaber-skill migration, which needed that published package before it could even start. Once the package was available, the closing dogfood ran in full and succeeded — see RESEARCH-0006 and lightsaber-skill#108. Status above moved from In Progress to Done once every "Gate-B closing dogfood" task confirmed passing, not before.

2026-08-18 — Codex review, PR #99: collision-preprocessing fix. TestDoubleAnalyzer's eligibleCandidates pass (diamond-collision and zero-argument-extension-collision detection) runs before the per-member emission loop's own FindImplementationForInterfaceMember guard, and was still including an already-resolved static abstract member's raw, IsAbstract: true symbol - reached via the closure walk visiting its declaring interface directly, not the resolved override. Since TestDoubleOverloadIdentity.CanonicalSignatureFor encodes only arity/parameter types (never return type or static-ness), a resolved zero-parameter static member sharing a name with a same-named zero- parameter instance member would be misclassified as a diamond-colliding identity, silently withholding the real instance member's Configure()/Verify() surface. eligibleCandidates now excludes any member that's IsStatic && IsAbstract && FindImplementationForInterfaceMember(...) is not null - the same condition the emission loop already uses to skip it. Regression test: StaticAbstractMemberResolvedByDerivedInterface_DoesNotCollideWithSameNamedInstanceMember.