Lyric

What changed in v2.16.0 — 2026-08-22

An interface may declare several parents. The rule that said otherwise rested on a claim about the runtime, and the claim was wrong.

Added

• Several parent interfaces: interface Item :: [Counted, Scaled]. Conforming to the child implies conforming to every ancestor, as before.

  The old rule read: a parent's default method needs its own slot indexes to remain valid behind a child-typed receiver, so several parents would need thunks. They do not. A vtable is keyed by the pair (concrete type, interface), and an implementing type gets a row for every interface in the transitive closure — every ancestor keeps its own numbering and nothing is remapped. The rule went when the reasoning was measured instead of repeated: lifted experimentally, probed, then built.

• What a second parent actually costs, and it is the second half of a rule that already existed (LYR-SEM0079): two parents may not contribute the same member name from DIFFERENT declarations. One slot holds one method, and no rule picks correctly between two. Before the check they were silently merged, which is worse than either answer.

  A name reached twice through a diamond is neither ambiguous nor refused: both paths lead to one declaration, so there is nothing to pick, and an implementation supplies it once.

Changed

• LYR-SEM0078 no longer includes "more than one parent"; LYR-SEM0079 gains the clash. Three conformance cases replace the one that pinned the old rule — including a default on the SECOND parent running behind a child receiver, which is the case the old reasoning said would break.

The full changelog →