Lyric

What changed in v3.6.0 — 2026-08-25

The toolchain twin of the specification round lyric-spec#21: the inference rules are written down in spec §8.3, and the two places where the compiler silently chose or silently accepted now say so out loud. No format change — the bytecode stays 3.6 — and no correct, order-independent program changes meaning.

Fixed

• The order of a :: list no longer decides a call (LYR-SEM0092). Type-argument inference through a conformance took the FIRST conformance in declaration order, so with Tag :: [Sink<int>, Sink<string>] the call pick(t, 42) compiled — and with the entries swapped it failed, complaining about a string nobody wrote. An argument that conforms to the wanted interface several times now refuses the inference, names every conformance it found, and points at the way out — the written type argument (pick<int>(t, 42)), which already wins over inference and needs no new machinery. One conformance, a conformance repeated across declarations, and a call with written type arguments behave as before.

• A :: list that repeats itself is refused (LYR-SEM0078 widened, the 2.15 precedent). struct S :: [Equatable<S>, Equatable<S>] compiled in silence — the list claimed two conformances where one exists. The rule reads the ENTRIES of one list: a parent written out beside its child ([Hashable<S>, Equatable<S>]) stays documenting style, and an extend restating a conformance the type also declares stays one conformance — the two declarations may live in different modules, and an upstream adoption must not break a downstream build. The parent list of an interface follows the same rule.

• catch (_: Boom) compiles instead of crashing the compiler (#115). A typed catch whose binding is _ — the exact form the unused-binding note recommends — died in the lowering with an internal error: the parser turns _ into "no name", the checker bound nothing, and the typed lowering path needs the binding for its TYPE. The clause is bound whenever a type is written; the name is scoped only when there is one. catch (_: SomeInterface) now reaches the documented refusal (LYR-IR0001) instead of the crash.

• A generic function used as a value is one sentence, not a cascade (LYR-SEM0052). let f = ident; bound f as fn(T) -> T and exploded at the use with "cannot assign 'T' to 'int'" — a T nobody wrote, reported where nobody wrote it. Function values are monomorphic (spec §8.1); the refusal now stands at the expression, with the way out: call it, or write a lambda that calls it.

Documentation

• Guide 7 stops claiming that v.mul(2.0) is ambiguous (it resolves by the overload rules since 3.0, exactly where v * 2.0 does) and that the arithmetic interfaces are "the one place" a type may conform twice (multi-conformance is general since 3.0). Both sentences had outlived the release that wrote them.
• Guide 8 gains inference through a conformance, the several-conformances refusal and its explicit-argument way out, and the rule that a generic function is not a function value.

The full changelog →