Scala 3.9

Apply the Java 25 prompt first. These rules cover Scala 3.9 LTS on the JVM, including its type-system escape hatches, implicit conversions, metaprogramming, concurrency, process APIs, and build inputs, and were checked against 3.9.0.

Validated types and authority

• Parse and validate external values before constructing case classes, enums, opaque types, or authorization objects; a Scala type proves only how a value was constructed in the paths you control.
• Give security-sensitive opaque types a validating smart constructor such as from or parse, keep the unchecked representation and constructor private, and do not expose unchecked apply, copy, or conversion paths that recreate invalid values.
• Model closed authorization states with enums or sealed hierarchies and make privileged matches exhaustive; do not let _, @unchecked, asInstanceOf, or a default allow branch hide a newly added case.
• Parse enum names through an explicit external-to-domain mapping. Do not persist or authorize by ordinal, and treat valueOf and fromOrdinal failures as invalid input rather than server errors.
• Keep contextual capabilities and security-sensitive given instances in the narrowest scope. Do not make tenant, identity, clock, key, or policy selection depend on an ambient instance that untrusted code can replace.
• Review every Conversion as executable code. A conversion must not validate by assertion, perform I/O, acquire authority, or silently turn untrusted data into a trusted domain type.
• Treat Scala 3.9's into[T] type and into modifier as an explicit