support multiple error responses for single status code

build.sc

@@ -161,7 +161,7 @@ object core extends BaseJavaModule {
 object protobuf extends BaseJavaModule {}
 
 val scalaVersionsMap =
-  Map("2.13" -> "2.13.7", "2.12" -> "2.12.17", "3" -> "3.3.0")
+  Map("2.13" -> "2.13.15", "2.12" -> "2.12.17", "3" -> "3.3.0")
 object openapi extends Cross[OpenapiModule](scalaVersionsMap.keys.toList)
 trait OpenapiModule extends BaseCrossScalaModule {
   val crossVersion = crossValue

modules/openapi/src/software/amazon/smithy/openapi/fromsmithy/protocols/AlloyAbstractRestProtocol.scala

@@ -120,11 +120,48 @@ abstract class AlloyAbstractRestProtocol[T <: Trait]
         createRequestBody(context, bindingIndex, operation)
           .foreach(builder.requestBody)
         createResponses(context, bindingIndex, operation)
-          .foreach { case (k, v) => builder.putResponse(k, v) }
+          .foreach { case (k, values) =>
+            combineResponseContent(values).foreach(v =>
+              builder.putResponse(k, v)
+            )
+          }
         Operation.create(method, uri, builder)
       })
       .asJava
 
+  def combineResponseContent(
+      responses: List[ResponseObject]
+  ): Option[ResponseObject] = {
+    responses match {
+      case Nil         => None
+      case head :: Nil => Some(head)
+      case head :: tail =>
+        val all = head +: tail
+        val mediaTypeObjects: List[MediaTypeObject] =
+          all.flatMap(_.getContent().asScala.toList.map {
+            case (_, mediaTypeObject) => mediaTypeObject
+          })
+        val schemas =
+          mediaTypeObjects.flatMap(_.getSchema().asScala)
+        val newSchema = Schema.builder().oneOf(schemas.asJava).build()
+        val newExamples = mediaTypeObjects
+          .flatMap(_.getExamples().asScala.toList)
+          .toMap
+          .map { case (k, exampleObj) => k -> exampleObj.toNode() }
+        val media = MediaTypeObject.builder
+          .examples(
+            newExamples.asJava
+          )
+          .schema(newSchema)
+          .build()
+        // `AlloyAbstractRestProtocol` only supports a single content-type, application/json
+        // This is why we can just use the content from `head` here
+        val newContent =
+          head.getContent().asScala.map { case (k, _) => k -> media }
+        Some(head.toBuilder().content(newContent.asJava).build())
+    }
+  }
+
   def createPathParameters(
       context: Context[T],
       operation: OperationShape
@@ -401,7 +438,7 @@ abstract class AlloyAbstractRestProtocol[T <: Trait]
     // operation shape.
     val updatedModel =
       context.getModel().toBuilder().addShape(operation).build()
-    val result = new util.TreeMap[String, ResponseObject]
+    val result = new util.TreeMap[String, List[ResponseObject]]
     val operationIndex = OperationIndex.of(updatedModel)
     operationIndex
       .getOutputShape(operation)
@@ -467,7 +504,7 @@ abstract class AlloyAbstractRestProtocol[T <: Trait]
       bindingIndex: HttpBindingIndex,
       operation: OperationShape,
       shape: StructureShape,
-      responses: util.Map[String, ResponseObject]
+      responses: util.Map[String, List[ResponseObject]]
   ) = {
     val operationOrError = reorganizeExampleTraits(operation, shape)
     val statusCode = context.getOpenApiProtocol.getOperationResponseStatusCode(
@@ -480,7 +517,14 @@ abstract class AlloyAbstractRestProtocol[T <: Trait]
       statusCode,
       operationOrError
     )
-    responses.put(statusCode, response)
+    val currentResponses: Option[List[ResponseObject]] = Option(
+      responses.get(statusCode)
+    )
+    val updatedResponses = currentResponses match {
+      case Some(current) => current :+ response
+      case None          => List(response)
+    }
+    responses.put(statusCode, updatedResponses)
   }
 
   private def createResponse(

modules/openapi/test/resources/foo.json

@@ -144,6 +144,11 @@
                     "in": "testinput"
                   }
                 },
+                "THREE": {
+                  "value": {
+                    "in": "testinputthree"
+                  }
+                },
                 "TWO": {
                   "value": {
                     "in": "testinputtwo"
@@ -177,9 +182,21 @@
             "content": {
               "application/json": {
                 "schema": {
-                  "$ref": "#/components/schemas/NotFoundResponseContent"
+                  "oneOf": [
+                    {
+                      "$ref": "#/components/schemas/NotFoundResponseContent"
+                    },
+                    {
+                      "$ref": "#/components/schemas/NotFoundTwoResponseContent"
+                    }
+                  ]
                 },
                 "examples": {
+                  "THREE": {
+                    "value": {
+                      "messageTwo": "Notfoundmessagetwo"
+                    }
+                  },
                   "TWO": {
                     "value": {
                       "message": "Notfoundmessage"
@@ -424,6 +441,17 @@
           "message"
         ]
       },
+      "NotFoundTwoResponseContent": {
+        "type": "object",
+        "properties": {
+          "messageTwo": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "messageTwo"
+        ]
+      },
       "SomeValue": {
         "oneOf": [
           {