Swift backend: toWire does not unwrap RawRepresentable (named string-union) fields, breaking wire round-trip #331

Summary

The Swift backend's provider wire-mapping path (toWire) does not unwrap RawRepresentable (named string-union / alias) fields — it stores the generated struct instead of its backing rawValue string. A wire round-trip (toWire → fromWire → load) then fails because the loader expects a String.

Discovered in microsoft/prompty after upgrading to @typra/emitter 2.1.8. The new toWire/fromWire wire-conversion tests added in 2.1.8 (fix for #328) surfaced this latent Swift-only bug.

Affected type

ModelOptions.reasoningEffort — the only field on that type with a named string-union alias type, rendered by the Swift backend as a RawRepresentable struct:

public struct ReasoningEffort: RawRepresentable, Equatable, Hashable, Codable {
  public let rawValue: String
  // .none/.minimal/.low/.medium/.high/.xhigh/.max ...
}

The bug

The plain save() path unwraps correctly:

// save() — CORRECT
if let value = self.reasoningEffort {
  result["reasoningEffort"] = value.rawValue
}

But the provider wire-mapping toWire() path does not:

// toWire() — BUG: stores the ReasoningEffort struct, not its String
if let wireKey = wireNameReasoningEffort, let value = self.reasoningEffort { result[wireKey] = value }

fromWire() maps the wire key back to the canonical field and calls load(), whose loader does TypraRuntime.string(value, field: "reasoningEffort"). Because the value is a ReasoningEffort struct rather than a String, it throws:

Expected string for field reasoningEffort.

Reproduction

Any schema with a ModelOptions-like model that has a named string-union alias field mapped to a provider wire name (e.g. @@knownAs(ModelOptions.reasoningEffort, "openai", "reasoning_effort")). The generated ModelOptionsTests.testWireConversion fails on Swift only; Python, C#, Rust, Go, Java, and TypeScript all pass (their toWire paths emit the backing string correctly).

Expected

The Swift toWire path should unwrap RawRepresentable/named-union fields via .rawValue, matching the save() path:

if let wireKey = wireNameReasoningEffort, let value = self.reasoningEffort { result[wireKey] = value.rawValue }

Scope

Swift backend only. Applies to any RawRepresentable-typed (named string-union / alias) field that participates in provider wire mapping — not just reasoningEffort.

Environment

• @typra/emitter 2.1.8
• Swift backend, format: false
• Consumer: microsoft/prompty (PR Live integration vectors, reasoningEffort, turn-coverage gate, and Typra 2.1.8 microsoft/prompty#522)

Current workaround

We patch the generated assignment to append .rawValue in a deterministic post-emit normalization step so the regenerate-no-diff gate stays green. Would prefer to drop the workaround once the emitter unwraps these fields in the Swift toWire path.