fix: restores nullable emission for v3

Signed-off-by: Vincent Biret <vibiret@microsoft.com>

Author: baywet

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -535,7 +535,7 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             writer.WriteOptionalCollection(OpenApiConstants.Enum, enumValue, (nodeWriter, s) => nodeWriter.WriteAny(s));
 
             // type
-            var serializedTypeProperty = TrySerializeTypeProperty(writer, version);
+            TrySerializeTypeProperty(writer, version);
 
             // allOf
             writer.WriteOptionalCollection(OpenApiConstants.AllOf, AllOf, callback);
@@ -580,13 +580,8 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             writer.WriteOptionalObject(OpenApiConstants.Default, Default, (w, d) => w.WriteAny(d));
 
             // nullable
-            if (version == OpenApiSpecVersion.OpenApi3_0 && serializedTypeProperty)
+            if (version == OpenApiSpecVersion.OpenApi3_0)
             {
-                // https://spec.openapis.org/oas/v3.0.4.html#fixed-fields-20
-                // This keyword only takes effect if type is explicitly defined within the same Schema Object.
-                //
-                // If the user explicitly set IsNullable to true, we serialize it even if redundant.
-                // But if **we** are inferring it (from oneOf/anyOf), we don't serialize it when it's redundant.
                 SerializeNullable(writer, version);
             }
 

src/Microsoft.OpenApi/Services/OpenApiV3_1ToV3SerializationConverter.cs

@@ -42,6 +42,7 @@ public override void Visit(IOpenApiSchema schema)
             {
                 JsonNullSentinel.JsonNull
             };
+            concreteSchema.Type = null;
         }
     }
 

test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.cs

@@ -964,7 +964,9 @@ public async Task SerializeOneOfWithNullAsV3ShouldUseNullableAsync()
                 """
                 {
                   "oneOf": [
-                    { },
+                    {
+                      "nullable": true
+                    },
                     {
                       "maxLength": 10,
                       "type": "string"
@@ -1004,7 +1006,9 @@ public async Task SerializeOneOfWithNullAndMultipleSchemasAsV3ShouldMarkItAsNull
                 """
                 {
                   "oneOf": [
-                    { },
+                    {
+                      "nullable": true
+                    },
                     {
                       "type": "string"
                     },
@@ -1052,7 +1056,9 @@ public async Task SerializeAnyOfWithNullAsV3ShouldUseNullableAsync()
                 """
                 {
                   "anyOf": [
-                    { },
+                    {
+                      "nullable": true
+                    },
                     {
                       "type": "object",
                       "properties": {
@@ -1095,7 +1101,9 @@ public async Task SerializeAnyOfWithNullAndMultipleSchemasAsV3ShouldApplyNullabl
                 """
                 {
                   "anyOf": [
-                    { },
+                    {
+                      "nullable": true
+                    },
                     {
                       "minLength": 1,
                       "type": "string"
@@ -1136,7 +1144,9 @@ public async Task SerializeOneOfWithOnlyNullAsV3ShouldJustBeNullableAsync()
                 """
                 {
                   "oneOf": [
-                    { }
+                    {
+                      "nullable": true
+                    }
                   ]
                 }
                 """;
@@ -1235,7 +1245,9 @@ public async Task SerializeOneOfWithNullAndRefAsV3ShouldUseNullableAsync()
                 """
                 {
                   "oneOf": [
-                    { },
+                    {
+                      "nullable": true
+                    },
                     {
                       "$ref": "#/components/schemas/Pet"
                     }
@@ -1610,7 +1622,7 @@ public void ApplySemanticConversionsFromV31ToV3ConvertsNullTypeToNullEnum()
 
             document.ApplySemanticConversions(OpenApiSpecVersion.OpenApi3_1, OpenApiSpecVersion.OpenApi3_0);
 
-            Assert.Equal(JsonSchemaType.Null, nullSchema.Type);
+            Assert.Null(nullSchema.Type);
             var enumValues = nullSchema.Enum;
             Assert.NotNull(enumValues);
             Assert.Single(enumValues);
@@ -2388,7 +2400,9 @@ public async Task SerializeNullableEnumWith3_0()
             var expected = """
                 {
                   "oneOf": [
-                    { },
+                    {
+                      "nullable": true
+                    },
                     {
                       "enum": [
                         "A",
@@ -2458,7 +2472,29 @@ public async Task SerializeNullableTypeWith3_0()
         {
             var schema = CreateTypeNullSchema();
             var result = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
-            var expected = "{ }";
+            var expected = """
+                {
+                  "nullable": true
+                }
+                """;
+
+            Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(result)));
+        }
+
+        [Fact]
+        public async Task SerializeMultipleTypesIncludingNullWith3_0EmitsNullable()
+        {
+            var schema = new OpenApiSchema
+            {
+                Type = JsonSchemaType.String | JsonSchemaType.Integer | JsonSchemaType.Null
+            };
+
+            var result = await schema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
+            var expected = """
+                {
+                  "nullable": true
+                }
+                """;
 
             Assert.True(JsonNode.DeepEquals(JsonNode.Parse(expected), JsonNode.Parse(result)));
         }