[release/9.0] Fix handling for inert route parameters in MVC endpoints for OpenAPI

src/OpenApi/src/Services/OpenApiDocumentService.cs

@@ -409,6 +409,10 @@ private async Task<OpenApiResponse> GetResponseAsync(
             var targetType = parameter.Type == typeof(string) && parameter.ModelMetadata.ModelType != parameter.Type
                 ? parameter.ModelMetadata.ModelType
                 : parameter.Type;
+            // If the type is null, then we're dealing with an inert
+            // route parameter that does not define a specific parameter type in the
+            // route handler or in the response. In this case, we default to a string schema.
+            targetType ??= typeof(string);
             var openApiParameter = new OpenApiParameter
             {
                 Name = parameter.Name,

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.ParameterSchemas.cs

@@ -592,4 +592,22 @@ internal enum ItemStatus
         Approved = 1,
         Rejected = 2,
     }
+
+    [Fact]
+    public async Task SupportsMvcActionWithAmbientRouteParameter()
+    {
+        // Arrange
+        var action = CreateActionDescriptor(nameof(AmbientRouteParameter));
+
+        // Assert
+        await VerifyOpenApiDocument(action, document =>
+        {
+            var operation = document.Paths["/api/with-ambient-route-param/{versionId}"].Operations[OperationType.Get];
+            var parameter = Assert.Single(operation.Parameters);
+            Assert.Equal("string", parameter.Schema.Type);
+        });
+    }
+
+    [Route("/api/with-ambient-route-param/{versionId}")]
+    private void AmbientRouteParameter() { }
 }