ignore DefaultMapping if V3

Author: kilifu

src/Microsoft.OpenApi/Models/OpenApiDiscriminator.cs

@@ -95,13 +95,6 @@ public void SerializeAsV3(IOpenApiWriter writer)
         {
             SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0);
 
-            // Write as x-oas-default-mapping extension in 3.0.0
-            if (DefaultMapping != null)
-            {
-                writer.WritePropertyName("x-oas-default-mapping");
-                DefaultMapping.SerializeAsV3(writer);
-            }
-
             writer.WriteEndObject();
         }
 

src/Microsoft.OpenApi/Reader/V3/OpenApiDiscriminatorDeserializer.cs

@@ -24,16 +24,7 @@ internal static partial class OpenApiV3Deserializer
                 }
             };
 
-        private static readonly PatternFieldMap<OpenApiDiscriminator> _discriminatorPatternFields = new() {
-                {s => s.StartsWith(OpenApiConstants.ExtensionFieldNamePrefix, StringComparison.OrdinalIgnoreCase), (o, p, n, doc) => {
-                        // Handle x-oas-default-mapping as DefaultMapping property
-                        if (p.Equals("x-oas-default-mapping", StringComparison.OrdinalIgnoreCase))
-                        {
-                            o.DefaultMapping = LoadMapping(n, doc);
-                        }
-                    }
-                }
-            };
+        private static readonly PatternFieldMap<OpenApiDiscriminator> _discriminatorPatternFields = new() {};
 
         public static OpenApiDiscriminator LoadDiscriminator(ParseNode node, OpenApiDocument hostDocument)
         {

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiDiscriminatorTests.cs

@@ -28,6 +28,7 @@ public async Task ParseBasicDiscriminatorShouldSucceed()
             var openApiDocument = new OpenApiDocument();
             var discriminator = OpenApiModelFactory.Load<OpenApiDiscriminator>(memoryStream, OpenApiSpecVersion.OpenApi3_0, OpenApiConstants.Yaml, openApiDocument, out var diagnostic, SettingsFixture.ReaderSettings);
 
+
             // Assert
             Assert.Equivalent(
                new OpenApiDiscriminator
@@ -38,8 +39,7 @@ public async Task ParseBasicDiscriminatorShouldSucceed()
                             ["puppy"] = new OpenApiSchemaReference("Dog", openApiDocument),
                             ["kitten"] = new OpenApiSchemaReference("Cat" , openApiDocument, "https://gigantic-server.com/schemas/animals.json"),
                             ["monster"] = new OpenApiSchemaReference("schema.json" , openApiDocument, "https://gigantic-server.com/schemas/Monster/schema.json")
-                    },
-                   DefaultMapping = new OpenApiSchemaReference("Animal", openApiDocument)
+                    }
                }, discriminator);
         }
     }

test/Microsoft.OpenApi.Readers.Tests/V3Tests/Samples/OpenApiDiscriminator/basicDiscriminator.yaml

@@ -3,4 +3,3 @@ mapping:
     puppy: '#/components/schemas/Dog'
     kitten: https://gigantic-server.com/schemas/animals.json#/components/schemas/Cat
     monster: https://gigantic-server.com/schemas/Monster/schema.json
-x-oas-default-mapping: '#/components/schemas/Animal'