ADR 0004: Lyth checker first, `go/types` validation second

Status: Accepted

Context

Diagnostics must be in Lyth terms at Lyth spans, but a young checker will miss cases, and silently passing invalid Go to go build would produce confusing errors.

Decision

1. The Lyth checker is the primary source of diagnostics.
2. After emission, each generated package is type-checked in-process with go/types, reusing the loaded dependency types. Errors are mapped through the exact source map and reported as E0902, with the original Go message kept.
3. Only then is go build invoked. Its errors (rare by then) are mapped through //line (E0903/E0904).

Alternatives

• Rely on go build only: text parsing, line-level positions, slow feedback.
• Trust the Lyth checker fully: unsafe while it matures.

Consequences

A safety net that turns checker gaps into located diagnostics. It costs a second, fast type check per package and keeps the checker honest in tests, because every golden program must pass validation.