Adds Attribute(store1, store2, ...) as a Tag-shaped array-part value
object that projects one or more Stores' named Source fields onto
native Roblox attributes in one binding, with .Merged for combining
heterogeneous Stores. Rejected: a bare [Attribute] = Store{...} hash
slot (only one slot per instance, can't merge heterogeneous stores)
and making Attribute a Store subtype (would let Store<T>'s T be a
Store again, colliding with the existing "handler-layer values can't
live in Source" rule). Verified the value-slot aggregation in .Merged
doesn't reopen the earlier "layered Store" rejection (archive/
context-rejected.md) since it's an explicit one-time author-time
composition, not an implicit read-time parent-chain fallback.
Renamed the existing single-key constructor Attribute<<T>>(name) to
AttributeKey<<T>>(name) to remove the name collision with the new
group primitive (provisional per existing OnChange/OnChangeKey
precedent; final bikeshed still queued in question.md).
Follow-up: AttributeKey(name) now memoizes through a name-keyed weak
table, guaranteeing AttributeKey(a) == AttributeKey(a) while any
strong reference (e.g. a live Dispatch chain entry) keeps it alive.
This let the group Attribute handler drop its originally-planned
self-contained SetAttribute/subscription logic and instead recurse
into the existing single-key AttributeKey dispatch path per field,
reusing its None/retract/store-bind handling instead of duplicating
it — the group handler's own state shrinks to just a prior-name-set
diff for deciding which keys to retractUnder. The same memoization
was applied to OnChangeKey for the same reason (pure name-to-key
mapping, no other varying state), confirmed safe even once
State<function> callbacks are involved.
Reflected across base/attribute-plan.md, base/onchange-plan.md,
base/architecture.md (source tree + Brand isX list), ROADMAP.md M2/M10,
question.md, README.md, and the luau-test type-narrowing spike.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
qwreey
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