ADR 0006 — OD-3: region parameters on aggregates and functions

• Status: Accepted (project owner, 2026-10-23: "implement OD-3 first", before C3). The two remaining questions of od3-design.md §10 were not answered explicitly; this ADR takes the design's defaults, which the owner may revisit.
• Scope: references and other scoped values inside user structs and enums, generic instances and built-in collections. Implemented as milestone M6x (milestone-6x.md).

Decision

1. Implement OD-3 now, as designed in od3-design.md, before C3.
2. Syntax (default of §10 question 2): region parameters <region r> on structs, enums and functions; region arguments first in a type's argument list (Token<src>, StringView<r>, Slice<r, T>); borrow(r) T for references; static is the built-in region of static data.
3. Precision (default of §10 question 3): one loan set per value, the union over its regions.
4. Regions are erased after checking. Two types that differ only in region arguments are the same type. Regions only determine, in signatures, which arguments' loans flow into the result and into borrow mut arguments (type-system.md §25.6).
5. Type parameters act as regions in signatures, and values whose type mentions a type parameter carry loans in generic bodies, so a generic checked once stays sound when instantiated with a scoped type.
6. Destroying a value that both holds loans and needs drop is a use of its loans (drop check). It reports the existing N0243 rather than a new code; N0271 stays unassigned.

Consequences

• E3 and E4 are relaxed exactly as type-system.md §25 says. Box, statics, constants, arrays, slices and pools still reject scoped values (N0246).
• v0 programs keep their meaning: a v0 signature has no region names, and its flows equal its v0 return sources.
• New diagnostics: N0270, N0272, N0273, N0274.