SumDispatch

The scrutinee-repr-driven half of a sum match. A MatchScaffolding captures the Case-builtin shape and any pre-bound state derived from the scrutinee (e.g. an unpacked field-list var, a tag binding) — but it does NOT decide the result repr. assembleMatch calls assemble once it has the aligned branches and the chosen target repr.

One scaffolding implementation per scrutinee repr (Term.Case for UplcConstr, CaseListLoweredValue for SumBuiltinList, …). The trait stays small so future per-repr emitters can implement it without pulling in the assembly machinery.

Branch-driven assembly. Universal across scrutinee reprs: upcasts each branch to the result type, runs the Phase-3b transparentSumUplcConstrAlignment override (or chooseCommonRepresentation as fallback) to pick the result repr, and asks the scaffolding to assemble the final value.

Choose the typegen that should emit this Constr, considering both the static type and surrounding-arg/context cues. Centralizes the rules formerly in ConstrDispatcher.shouldDelegateToUplcConstr:

1. Parent sum's default representation is SumUplcConstr → use parent's gen.
2. The constr is List/Option-shaped AND either: a. it's a List.Cons whose tail's actual representation is already SumUplcConstr (spine propagation), or b. lctx.inUplcConstrListScope == true (caller asked for native lists).
3. Caller's optTargetType is @UplcRepr(UplcConstr)-annotated.

Returns the typegen to dispatch to; default is the static-type's own typegen.

Pick the parent-sum repr that an upcastOne from input to targetType should produce — the "preserve concrete vs coerce to default" decision:

• ProdUplcConstr input → overlay the variant onto buildSumUplcConstr(targetType) so concrete field reprs (e.g. Unwrapped element types that differ from the DataDecl's abstract defaults) survive the upcast. Avoids a Data round-trip that fails for abstract TypeVar fields in isolation. Fires unconditionally; don't gate on target's default repr.
• SumUplcConstr input → keep the refined variant set as-is.
• Same-shape sum input (SumBuiltinList, SumPairBuiltinList, PackedSumDataList, SumDataAssocMap) → keep input's parameterization (preserves element repr).
• Otherwise → target's default repr.

Returns the parent-sum repr only. Each emitter still chooses the matching byte-level conversion (ProdDataList → SumBuiltinList, ProdDataConstr → DataConstr, etc.) before relabeling.

Pick the type generator for a Nil constructor.

Lowering.lowerSIR routes Nil straight to typeGenerator.genConstrLowered, bypassing chooseConstrOutputEmitter. So this path also has to apply the inUplcConstrListScope override (the rule that, pre-Phase-3a, the typegen- internal ConstrDispatcher.shouldDelegateToUplcConstr rule #4 supplied); without it, a Nil produced inside an UplcConstr-scoped intrinsic body whose surrounding target type is unannotated List[A] would emerge as SumBuiltinList, desyncing from match-branch SumUplcConstr reprs.

Returns (generator, effective-constr): effective-constr rewrites constr.tp to targetType when one is supplied so the generator sees the right type info for type-correct Nil emission.

Representation-aware dispatch for sum-typed genMatch. The Case-builtin shape varies by loweredScrutinee.representation:

• SumUplcConstr → tag-ordered Case via genMatchUplcConstr (the scrutinee's UPLC bytes are already a Constr(tag, fields) so branches index by tag directly).
• DataConstr → Data-shape Case in DataConstrEmitter. genMatchDataConstr (unConstrData → tag/field-list → if/else chain, or Case on integer for V4+).
• SumBuiltinList(elemRepr) → caseList-ordered Case via the element-repr-parameterized SumBuiltinListEmitter.genMatch.
• PackedSumDataList → unpack to SumBuiltinList(<elem-default>), bind a fresh var, recurse.
• TypeVarRepresentation(_) → relabel to the type's defaultTypeVarRepresentation and recurse.
• everything else → fall back to the type-keyed typegen's genMatch (same routing as before this dispatcher existed).

Pre-Phase-4 this match lived inside DataConstrEmitter.genMatch (formerly SumCaseSirTypeGenerator) and the SumUplcConstr short-circuit lived in Lowering.lowerSIR; routing them through the dispatcher consolidates the choice.

Choose-and-align step for SumUplcConstr matches when any branch carries a (Sum|Prod)UplcConstr whose field reprs include a Transparent TypeVar. Returns Some((sumRepr, aligned)) when the override fires, None otherwise (caller should fall back to its own convergence — typically LoweredValue.chooseCommonRepresentation).

The override fires because Transparent TypeVar fields cannot be folded to a Data-shaped repr — we must keep a SumUplcConstr shape and align branches structurally. Alignment: (Sum|Prod)UplcConstr branches and ErrorRepresentation stay as-is; others route through toRepresentation(sumRepr, pos) for Data→UplcConstr conversion.

The synthesized SumUplcConstr uses buildSumUplcConstr's default variants for tags without a matching branch puc; hand-rolled defaultRepresentation lookups would produce TypeVarRepresentation(Fixed) (because the DataDecl's A carries Fixed kind), which then leaks into downstream inference and surfaces as Fixed → Unwrapped aborts.