use default literal

src/Microsoft.OpenApi.Readers/OpenApiTextReaderReader.cs

@@ -104,7 +104,7 @@ public T ReadFragment<T>(TextReader input, OpenApiSpecVersion version, out OpenA
             {
                 diagnostic = new OpenApiDiagnostic();
                 diagnostic.Errors.Add(new OpenApiError($"#line={ex.Start.Line}", ex.Message));
-                return default(T);
+                return default;
             }
 
             return new OpenApiYamlDocumentReader(this._settings).ReadFragment<T>(yamlDocument, version, out diagnostic);

src/Microsoft.OpenApi.Readers/ParseNodes/MapNode.cs

@@ -74,7 +74,7 @@ public override Dictionary<string, T> CreateMap<T>(Func<MapNode, T> map)
                     {
                         Context.StartObject(key);
                         value = n.Value as YamlMappingNode == null
-                          ? default(T)
+                          ? default
                           : map(new MapNode(Context, n.Value as YamlMappingNode));
                     } 
                     finally

src/Microsoft.OpenApi.Readers/ParsingContext.cs

@@ -158,10 +158,10 @@ public T GetFromTempStorage<T>(string key, object scope = null)
             }
             else if (!_scopedTempStorage.TryGetValue(scope, out storage))
             {
-                return default(T);
+                return default;
             }
 
-            return storage.TryGetValue(key, out var value) ? (T)value : default(T);
+            return storage.TryGetValue(key, out var value) ? (T)value : default;
         }
 
         /// <summary>

test/Microsoft.OpenApi.Tests/Visitors/InheritanceTests.cs

@@ -20,7 +20,7 @@ public void ExpectedVirtualsInvolved()
 
 			visitor = new TestVisitor();
 
-			visitor.Enter(default(string));
+			visitor.Enter(default);
 			visitor.Visit(default(OpenApiDocument));
 			visitor.Visit(default(OpenApiInfo));
 			visitor.Visit(default(OpenApiContact));