W-ENGINE-INITIAL-MISSING

What it means

An <engine> must commit to a starting variant. Per §51.0.E, the initial= attribute is REQUIRED on every non-derived engine. When omitted, the compiler will not refuse compilation — it falls back to the first state-child's variant declaration order — but emits W-ENGINE-INITIAL-MISSING to surface the implicit commitment.

Adopters using the prototyping shape (rapid sketch where state order in the source IS the intended initial) get a working engine. Adopters who care about explicit authoring intent get a nudge to declare initial=.Variant.

Minimal reproducer

type Phase:enum = { Idle, Loading, Done }

<engine for=Phase>
//      ^^^^^^^^^
//      W-ENGINE-INITIAL-MISSING: initial= absent.
//      Defaults to .Idle (first state-child).
  <Idle    rule=.Loading />
  <Loading rule=.Done    />
  <Done    rule=.Idle    />
</>

How to fix

1. Add initial=.Variant — the canonical form. Makes the starting variant explicit, survives state-child reordering without behavior change, and silences the warning.
2. Suppress the warning per SPEC §28 lint suppression configs if the prototype shape is intentional. Config: lint.engine-initial-missing. Use sparingly; explicit initial= is preferred for production code.

Derived engines: initial= is FORBIDDEN

Derived engines (per §51.0.J, <engine for=Type derived=expr>) reject initial= entirely — the starting variant is whatever expr evaluates to at module-init time, NOT an authored value. Providing initial= on a derived engine fires E-DERIVED-ENGINE-NO-INITIAL (error, not warning).

Related

• <engine> — element reference.
• E-DERIVED-ENGINE-NO-INITIAL — error for the opposite case (initial= on derived engine).

Specification

Normative text: SPEC §51.0.E (initial= required on non-derived engines + first-state-child fallback rule), §51.0.J (derived engines reject initial=), §28 (lint suppression configs), §34 catalog row. Spec lives at compiler/SPEC.md .