simplify, add Validator derivation and reuse, add Validator tests

core/src/main/scala-3/sttp/tapir/macros/CodecMacros.scala

@@ -40,9 +40,14 @@ trait CodecMacros {
   inline def derivedEnumerationValueCustomise[L, T <: scala.Enumeration#Value]: CreateDerivedEnumerationCodec[L, T] =
     new CreateDerivedEnumerationCodec(derivedEnumerationValueValidator[T], SchemaAnnotations.derived[T])
 
+  /** Creates a codec for a string-based union of constant values, where the validator is derived using
+    * [[sttp.tapir.Validator.derivedStringBasedUnionEnumeration]]. This requires that the union is a union of string literals.
+    *
+    * @tparam T
+    *   The type of the union.
+    */
   inline given derivedStringBasedUnionEnumeration[T](using IsUnionOf[String, T]): Codec[String, T, TextPlain] =
-    lazy val values = UnionDerivation.constValueUnionTuple[String, T]
-    lazy val validator = Validator.enumeration(values.toList.asInstanceOf[List[T]])
+    lazy val validator = Validator.derivedStringBasedUnionEnumeration[T]
     Codec.string.validate(validator.asInstanceOf[Validator[String]]).map(_.asInstanceOf[T])(_.asInstanceOf[String])
 
   /** A default codec for enumerations, which returns a string-based enumeration codec, using the enum's `.toString` to encode values, and

core/src/main/scala-3/sttp/tapir/macros/SchemaCompanionMacrosExtensions.scala

@@ -7,9 +7,8 @@ object SchemaCompanionMacrosExtensions extends SchemaCompanionMacrosExtensions
 
 trait SchemaCompanionMacrosExtensions:
   inline given derivedStringBasedUnionEnumeration[S](using IsUnionOf[String, S]): Schema[S] =
-    lazy val values = UnionDerivation.constValueUnionTuple[String, S]
-    lazy val validator = Validator.enumeration(values.toList.asInstanceOf[List[S]])
+    lazy val validator = Validator.derivedStringBasedUnionEnumeration[S]
     Schema
       .string[S]
-      .name(SName(values.toList.mkString("_or_")))
+      .name(SName(validator.possibleValues.toList.mkString("_or_")))
       .validate(validator)

core/src/main/scala-3/sttp/tapir/macros/ValidatorMacros.scala

@@ -13,6 +13,11 @@ trait ValidatorMacros {
     * first place (the decoder has no other option than to fail).
     */
   inline def derivedEnumeration[T]: Validator.Enumeration[T] = ${ ValidatorMacros.derivedEnumerationImpl[T] }
+
+  inline def derivedStringBasedUnionEnumeration[T](using IsUnionOf[String, T]): Validator.Enumeration[T] = {
+    lazy val values = UnionDerivation.constValueUnionTuple[String, T]
+    Validator.enumeration(values.toList.asInstanceOf[List[T]])
+  }
 }
 
 private[tapir] object ValidatorMacros {

core/src/test/scala-3/sttp/tapir/ValidatorScala3EnumTest.scala

@@ -15,6 +15,16 @@ class ValidatorScala3EnumTest extends AnyFlatSpec with Matchers {
     """)
   }
 
+  it should "derive a validator for a string-based union type" in {
+    // given
+    val validator = Validator.derivedStringBasedUnionEnumeration["Apple" | "Banana"].asInstanceOf[Validator.Primitive[String]]
+
+    // then
+    validator.doValidate("Apple") shouldBe ValidationResult.Valid
+    validator.doValidate("Banana") shouldBe ValidationResult.Valid
+    validator.doValidate("Orange") shouldBe ValidationResult.Invalid()
+  }
+
 }
 
 enum ColorEnum {