[OmarAlJarrah]

Summary

Closes #1686.

An inline allOf branch says what it writes for the position as if written beside the allOf, so its description, deprecation, examples, extensions, bounds and use-bound keywords fold where the same keyword beside the allOf lands (docs/compilers/openapi.md#allof-inline-residue). Beside a oneOf, an anyOf or a $dynamicRef, a branch folded nothing. It stayed whole under Unmodeled["openapi:allOf/<n>"]: on every composed variant where the union distributes, and on the Model where the union or the $dynamicRef is kept verbatim. The issue's example:

S:
  allOf: [{description: what S is, x-sensitive: true, properties: {c: {type: string}}}]
  oneOf: [{$ref: '#/components/schemas/A'}, {$ref: '#/components/schemas/B'}]

On main, S lowers to a Union with an empty Docs.Description, and each variant keeps the branch whole. With description and x-sensitive written beside the allOf, the same schema puts both on the Union. JSON Schema reads the two spellings alike, but they lowered apart.

Decision

The branches fold. A variant is the conjunction, so what a branch says of the position is said of each variant. Folding puts it in one place, where the beside spelling already puts it:

• docs, deprecation, examples, secrecy and XML hints on the Union;
• what a use binds on its carriers, or kept verbatim on the Union where none uses it;
• a value bound on each composed variant, since it constrains both sides of the disjunction.

A branch with no residue is then kept on no variant. Keeping the branch whole on each variant let any one variant rebuild the source, but it copied one fact onto nodes no reader of the position looks at, and it made two spellings that mean the same thing lower differently. docs/ir/models.md#composition-beside-a-union records the choice and the rejected reading.

Change

• allOfDecides (voices.go) took over allOfConjoins' exclusion of a union and a $dynamicRef beside the allOf. Those exclusions belong to refinement, which keeps composing beside either (allof-refinement, cases 1 and 4). The composition is still what the position lowers, so allOfDecides now excludes only a $ref beside the allOf, whose $ref site keeps the allOf verbatim. allOfConjoins keeps all three exclusions and is renamed allOfMayRefine, since refinement is all it now answers for.
• readAllOf marks which branches fold for both composers, lowerAllOf and the distributed variants. It moves beside allOfBranches in allof.go.
• buildComposedVariant reads the folding branches' bounds, as lowerAllOf does, from voices lowerDistributedUnion reads once for every variant, beside the flattened allOf composedBody already carries.
• A use through a $ref reads what the folding branches lend (lendingBranches), so a property naming such a schema takes the branch's readOnly, default, docs, deprecation and secrecy, as it takes them written beside the allOf.
• $dynamicRef is in scope because it is the same predicate clause, and the same divergence shows there: {$dynamicRef: '#meta', allOf: [{$ref: Obj}, {title: t, maxProperties: 3}]} kept the branch whole, while title and maxProperties beside the allOf reached Docs and Constraints.

What does not change: a union or $dynamicRef beside the allOf still keeps it from refining. An enum or const beside both still wins the election and keeps the allOf verbatim. A branch that disagrees with a conjunct, or reads its $refs against another base, still stays whole with its report.

Effect on real input

• Golden spec (@fb54d32f): IR and every diagnostic are identical to main's apart from irVersion. It writes no folding branch beside a union today. The five filters/items request-body positions openapi: decide whether a composition distributed across a union folds its branches at the union #1686 names reach this rule only once the branch spelling of openapi: a union written as an allOf branch is kept verbatim, though beside the body it distributes #1682 lowers as a union beside the body.
• The 1,954 real-world OpenAPI and Swagger documents sweep.py reads from the reference corpus, plus the 237 specs under testdata: two documents change. One is the new conformance case. In the other, a schema written {allOf: [{multipleOf: 2}], anyOf: […], oneOf: […]} now holds multipleOf: 2 in its Model's Constraints, as the same bound written beside the allOf does on main.
• Cost: compiling a union of 500 $ref variants beside an allOf of a $ref and 20 inline branches takes a quarter fewer instructions than on main where the inline branches fold (medians of five runs, 1,068M against 1,425M), since no variant copies them, and as many where they do not (1,965M against 1,967M).

Breaking

IRVersion moves to 0.38.0 with no change of shape (docs/ir/version-history.md). A consumer pinned to 0.37.0 that read such a branch under Unmodeled["openapi:allOf/<n>"] no longer finds it there; it finds what the branch said on the Union, its carriers, each variant's Constraints or the Model.

Test plan

At head d480c86:

• TestVoices_AKeywordFoldsWhereTheAllOfWouldKeepIt compiles 22 keywords on a branch and beside the allOf, at five outcomes: alone, distributed, beside a degraded union (a oneOf beside an anyOf), beside a narrowing union and beside a $dynamicRef, each row first asserting the report that shows it reached its outcome. It compares the position's node, each variant and a component holding a $ref use of the position whole with cmp, but for the pointer, position and node a provenance places it at, and the diagnostics by severity and code. With allOfDecides given back the union and $dynamicRef exclusions, all 66 union rows and all 22 $dynamicRef rows fail while the 22 alone rows pass. With the variant's constraints read without voices, the distributed maxProperties row fails. With the fold marks cleared for the distributed composition only, the 22 distributed rows fail. With lendingBranches given back the same exclusions, the readOnly, default, title, description, externalDocs, deprecated and redaction rows fail at each union and $dynamicRef outcome.
• TestVoices_KeptAndMetBranchesHoldAtEveryOutcome runs, at the same five outcomes, a branch disagreeing with a conjunct and two meeting maxProperties, each in both orders, and a branch reading a $ref against another base, holding each Model the composition lowers to. A voicing that let a disagreeing branch fold, ignored another base, or kept two meeting bounds apart, each only where a union or a $dynamicRef stands beside the allOf, passed every test under compilers/openapi before it. Each now fails its rows at every union and $dynamicRef outcome, and passes alone.
• TestVoices_AUseBindingKeywordReachesItsCarrierOrItsNode gains a distributed carrier: the branch's readOnly and default reach the property. TestNullableWidening_ABranchWidensTheAllOfHoldingIt now holds a 3.0 branch nullable: true beside a degraded union (a oneOf beside an anyOf) and a distributed one to widen the use, as nullable: true beside the allOf already did on main. Each row asserts the outcome its name claims. TestAllOf_BranchResidueRidesEveryDistributedVariant keeps a residue keyword and asserts the description also reaches the Union.
• testdata/conformance/openapi/allof-inline-residue.yaml gains Distributed. Its golden shows the description on the Union, maxProperties: 4 on each variant, and no residue report.
• Beside and branch spellings compiled side by side at eleven schema positions (a component, a property, a $ref use, items, additionalProperties, prefixItems, an inline union branch, a parameter, a header, a request body and a response body), with where provenance places a node projected away: 33 keyword rows under 3.1 and 12 under 3.0, one in each writing several keywords together, at nine outcomes (alone, distributed by a oneOf and by an anyOf, both combinators, a narrowing union, an inline union branch, an unresolved branch, a discriminator, and a $dynamicRef under 3.1 only). Each branch spelling's IR and diagnostic codes equal its beside spelling's, but for the keywords that do not fold alone either ($comment, a format other than a redaction, minItems, uniqueItems, unevaluatedProperties: false); every beside spelling's equals main's; and a branch spelling's differs from main's only beside a union or a $dynamicRef.
• go run ./cmd/morphic-harness passes on the new case and on every branch spelling above but the unresolved ones, which stop at their error by design. Reversing each spec's component order changes no document, operations included, and writing the folding branch before the $ref moves only a kept branch's allOf/<n> key, as it does with no union. go test with own-package coverage (100%), go vet and golangci-lint run on compilers/openapi/internal/schema, TestDocuments and TestDocComments in internal/archtest, TestLoweringSpec, and scripts/vet-rules.sh are clean; the full gate is CI's.
• CI gate on the head.

The degraded fixtures write a oneOf beside an anyOf rather than a union with an inline branch, since distributing inline branches is planned (#661) and would turn those rows into a second distributed case. The subject of commit 3766457 is 73 characters, one over the cap. The branch is pushed, so it stays, and the squash body set at merge keeps it off main.