IsNominalType

A compile-time witness that A is a nominal type.

A nominal type is any fully-known, concrete named type: a class, trait, object, enum, or applied type constructor (e.g. List[Int]). Abstract type parameters, union types (A | B), intersection types (A & B), structural refinements, and type lambdas are not nominal and will cause derivation to fail with a compile error.

IsNominalType[A] is automatically derived by a macro when A is nominal. The key effect of requiring IsNominalType[A] in a method signature is that it prevents the method from being called with an unresolved type parameter, ensuring the type is concrete at the call site.