Refactor EraseUnions: DU-based domain model for union lowering#19518

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T-Gro wants to merge 45 commits intomainfrom

refactoring/erase-unions

Commits on Mar 30, 2026

Add UnionLayout taxonomy + assertions (no behavior change)

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Extract shared concerns (nullability, debug proxy)

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Migrate instruction functions to exhaustive layout matching

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Decompose convAlternativeDef into focused sub-functions

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Decompose mkClassUnionDef into focused sub-functions

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Remove old UnionReprDecisions infrastructure

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Refactor mkNewData: replace boolean deconstruction with pattern match

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Deduplicate tyForAlt by reusing altOptimizesToRoot

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Simplify processAlternative NoHelpers branch with pattern match

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Simplify emitTesterMethodAndProperty nullness guard

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Simplify emitRootClassFields loop guard with layout match

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Decompose emitRootConstructors complex guard into named conditions

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Replace isStruct bool with layout match in rewriteFieldsForStructFlattening

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Eliminate isSingleNonNullaryFoldedToRoot in favor of altFoldsAsRootInstance

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Eliminate caseFoldsToRootClass, inline via altFoldsAsRootInstance

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Refactor classifyUnion: use match expression for readable classification

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Simplify maintainConstantField: replace chained AP-to-bool with match

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Simplify emitNullaryCaseAccessor: match on CaseIsNull directly

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Apply fantomas formatting

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Wave 1: Replace avoidHelpers bool with DataAccess DU

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Wave 2: Rename nullCaseIdx→nullAsTrueValueIdx, ListTailOrNull→FSharpList

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Wave 3: Split allNullary bool into explicit TaggedRef/TaggedRefAllNullary DU cases

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Wave 3+4: Split allNullary and SmallRefUnion into explicit DU cases

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Wave 5: Remove dead code, eliminate hasNullCase, explicit match arms

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Wave 6: Extract nullnessCheckingEnabled helper, restructure classifyUnion

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Wave 7: Inline doesRuntimeTypeDiscriminateUseHelper, match-based tester guard

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Wave 8: Split TaggedStructAllNullary from when guard, remove dead fallthrough

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F10+F11: Remove duplicate comment, clean unreachable match arm

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Round 2: Domain model improvements (F1+F5+F7+F8+F4)

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Round 2b: CaseIdentity, CaseStorage broadened, renames, dedup

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Perf: struct CaseIdentity, inline CaseIsNull AP, eliminate allocations

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Eliminate redundant classifyFromSpec/baseTyOfUnionSpec in EraseUnions

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Readability: extract nullable rewrite helper, improve comments

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Phase 4: emitIsCase+emitBranchOnCase → CaseStorage × DiscriminationMethod

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Phase 4: Architecture docs, InteropCapability, reduce mkMethodsAndProps params

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Remove speculative InteropCapability code

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Remove dead code: _validateActivePatterns, NonNullaryFoldsToRoot, FieldsOnRootType APs

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Enrich DiscriminationMethod AP to carry baseTy and nullAsTrueValueIdx

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Split EraseUnions.fs into 3 files by semantic boundary

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Fix bug + review findings: bounds check, stale comments, magic numbers

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DDD review: pure classification in Types.fs, architecture docs, section headers

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Council fixes: private mkMethodsAndPropertiesForFields, shared adjustFieldNameForList

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Fix: preserve exact IL for Option — add DataAccess.ViaOptionHelpers

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Commits on Mar 31, 2026