ADR-015: Runtime Formatter Subset Diagnostics

Palamedes runtime formatting intentionally covers a small ICU formatter subset that maps directly to platform Intl APIs. The current runtime can render plain number, date, and time arguments plus a few common styles, but it does not implement every formatter kind or ICU skeleton that translators may write in PO files.

Status: Accepted Date: 2026-06-12

Context

Before this decision, catalog artifact compilation validated ICU syntax and source/translation compatibility, but it did not make the runtime formatter subset explicit. Unsupported formatter kinds could reach runtime artifacts without a targeted diagnostic, and unsupported styles were hard to distinguish from fully invalid ICU.

Decision

Palamedes validates final compiled catalog artifacts against the runtime formatter subset after Ferrocat compiles the artifact.

The supported runtime subset is:

number with no style
number with percent or integer
number with ::percent, ::integer, or ::currency/ISO
date and time with no style
date and time with short, medium, long, or full

Currency formatting intentionally requires the ::currency/ISO skeleton form; bare currency/ISO is treated as outside the runtime formatter subset.

Palamedes treats unsupported formatter kinds such as list, duration, ago, and name as errors because the runtime parser does not render those formatter kinds. Unsupported styles on supported formatter kinds are warnings because the runtime currently falls back to the default Intl formatter for that argument type.

Generic ICU formatter support analysis lives in Ferrocat. Palamedes supplies only the runtime-specific policy and maps Ferrocat diagnostics back to compiled artifact source keys and resolved locales.

This PR does not add standalone i18n.formatNumber() or i18n.formatDate() helpers. Those can be evaluated separately if application authors need direct formatting APIs outside translated message patterns.

Alternatives Considered

1. Implement formatter validation entirely inside Palamedes

Rejected because identifying ICU formatter references is generic MessageFormat analysis. Keeping that analysis in Ferrocat preserves the catalog boundary from ADR-006 and avoids a second local ICU walker.

2. Treat unsupported styles as errors

Rejected for now because the runtime still renders supported formatter kinds by falling back to default Intl options. A warning reflects current behavior and lets teams decide whether unsupported style fallback is acceptable.

3. Expand the runtime to full ICU skeleton support first

Rejected as larger scope. The immediate need is to make the existing runtime contract explicit and reviewable in catalog artifacts.

Consequences

Catalog artifact diagnostics now report runtime-incompatible formatter kinds with icu.unsupported_formatter_kind.
Catalog artifact diagnostics now report unsupported styles on supported runtime formatter kinds with icu.unsupported_formatter_style.
Diagnostics use the compiled artifact source key and the locale that provided the final runtime message, including source-locale fallbacks for missing translations.
Future runtime formatter expansion should update this ADR, the Core runtime docs, and the Palamedes runtime policy together.