From e5f9e9bc0cb5c1b801640600887f67c72b5e5687 Mon Sep 17 00:00:00 2001
From: david-perez <d@vidp.dev>
Date: Fri, 24 May 2024 18:26:20 +0200
Subject: [PATCH] Servers must reject `{}` when deserializing a union

Once we're inside of a `{}` JSON object, a known union variant must be
provided. Servers must not accept `{}` as "the union shape was not set"
when the union shape is optional (which was the behavior in smithy-rs
prior to https://github.com/smithy-lang/smithy-rs/pull/3481).

This commit adds a protocol test to pin this behavior.
---
 .../malformedRequests/malformed-union.smithy  | 22 +++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/smithy-aws-protocol-tests/model/restJson1/malformedRequests/malformed-union.smithy b/smithy-aws-protocol-tests/model/restJson1/malformedRequests/malformed-union.smithy
index cd0ca8cd477..ab4571cdb51 100644
--- a/smithy-aws-protocol-tests/model/restJson1/malformedRequests/malformed-union.smithy
+++ b/smithy-aws-protocol-tests/model/restJson1/malformedRequests/malformed-union.smithy
@@ -78,6 +78,28 @@ apply MalformedUnion @httpMalformedRequestTests([
             }
         }
     },
+    {
+        id: "RestJsonMalformedUnionEmptyObjectNoFieldsSet",
+        documentation: """
+            When the union is an empty object, it has no fields set, so the
+            response should be a 400 SerializationException.""",
+        protocol: restJson1,
+        request: {
+            method: "POST",
+            uri: "/MalformedUnion",
+            body: """
+                { "union" : {  } }""",
+            headers: {
+                "content-type": "application/json"
+            }
+        },
+        response: {
+            code: 400,
+            headers: {
+                "x-amzn-errortype": "SerializationException"
+            }
+        }
+    },
     {
         id: "RestJsonMalformedUnionValueIsArray",
         documentation: """