OpenAPI/ApiExplorer: Fix paths without binding in Minimal Api's not discovered

src/Mvc/Mvc.ApiExplorer/src/EndpointMetadataApiDescriptionProvider.cs

@@ -114,6 +114,7 @@ private ApiDescription CreateApiDescription(RouteEndpoint routeEndpoint, string
 
         var hasBodyOrFormFileParameter = false;
         var parameters = routeEndpoint.Metadata.GetOrderedMetadata<IParameterBindingMetadata>();
+        var remainingRouteParameters = routeEndpoint.RoutePattern.Parameters.ToDictionary(p => p.Name, StringComparer.OrdinalIgnoreCase);
 
         foreach (var parameter in parameters)
         {
@@ -122,13 +123,35 @@ private ApiDescription CreateApiDescription(RouteEndpoint routeEndpoint, string
             if (parameterDescription is { })
             {
                 apiDescription.ParameterDescriptions.Add(parameterDescription);
+                if (parameterDescription.Source == BindingSource.Path)
+                {
+                    remainingRouteParameters.Remove(parameterDescription.Name);
+                }
 
                 hasBodyOrFormFileParameter |=
                     parameterDescription.Source == BindingSource.Body ||
                     parameterDescription.Source == BindingSource.FormFile;
             }
         }
 
+        // Add any remaining route parameters that weren't associated with a parameter in the delegate.
+        foreach (var remainingRouteParameter in remainingRouteParameters.Values)
+        {
+            var parameterDescription = new ApiParameterDescription
+            {
+                Name = remainingRouteParameter.Name,
+                Source = BindingSource.Path,
+                IsRequired = true,
+                RouteInfo = new ApiParameterRouteInfo
+                {
+                    IsOptional = false,
+                    Constraints = Array.Empty<IRouteConstraint>(),
+                    DefaultValue = null,
+                },
+            };
+            apiDescription.ParameterDescriptions.Add(parameterDescription);
+        }
+
         // Get IAcceptsMetadata.
         var acceptsMetadata = routeEndpoint.Metadata.GetMetadata<IAcceptsMetadata>();
         if (acceptsMetadata is not null)

src/Mvc/Mvc.ApiExplorer/test/EndpointMetadataApiDescriptionProviderTest.cs

@@ -767,6 +767,33 @@ static void AssertPathParameter(ApiDescription apiDescription)
         AssertPathParameter(GetApiDescription((TryParseStringRecordStruct foo) => { }, "/{foo}"));
     }
 
+    [Fact]
+    public void AddsFromRouteParameterAsPathWhenOmittedInHandler()
+    {
+        static void AssertPathParameter(ApiDescription apiDescription)
+        {
+            var param = Assert.Single(apiDescription.ParameterDescriptions);
+            Assert.Equal("foo", param.Name);
+            Assert.Equal(BindingSource.Path, param.Source);
+        }
+
+        AssertPathParameter(GetApiDescription(() => { }, "/{foo}"));
+    }
+
+    [Fact]
+    public void AddsFromRouteParameterAsPathOnceRegardlessOfCasing()
+    {
+        static void AssertPathParameter(ApiDescription apiDescription)
+        {
+            var param = Assert.Single(apiDescription.ParameterDescriptions);
+            Assert.Equal(typeof(int), param.Type);
+            Assert.Equal(typeof(int), param.ModelMetadata.ModelType);
+            Assert.Equal(BindingSource.Path, param.Source);
+        }
+
+        AssertPathParameter(GetApiDescription((int foo) => { }, "/{FOO}"));
+    }
+
     [Fact]
     public void AddsFromQueryParameterAsQuery()
     {

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.SourceGenerators.Tests/OperationTests.MinimalApis.cs

@@ -530,4 +530,45 @@ await SnapshotTestHelper.VerifyOpenApi(compilation, document =>
             Assert.Equal("Property with only value documentation.", valueOnlyParam2.Description);
         });
     }
+
+    [Fact]
+    public async Task SupportsRouteParametersFromMinimalApis()
+    {
+        var source = """
+using System;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.AspNetCore.Http;
+
+var builder = WebApplication.CreateBuilder();
+
+builder.Services.AddOpenApi();
+
+var app = builder.Build();
+
+app.MapGet("/{userId}", RouteHandlerExtensionMethods.Get);
+
+app.Run();
+
+public static class RouteHandlerExtensionMethods
+{
+    /// <param name="userId">The id of the user.</param>
+    public static string Get()
+    {
+        return "Hello, World!";
+    }
+}
+""";
+
+        var generator = new XmlCommentGenerator();
+        await SnapshotTestHelper.Verify(source, generator, out var compilation);
+        await SnapshotTestHelper.VerifyOpenApi(compilation, document =>
+        {
+            var path = document.Paths["/{userId}"].Operations[HttpMethod.Get];
+            Assert.NotEmpty(path.Parameters);
+            Assert.Equal("The id of the user.", path.Parameters[0].Description);
+        });
+    }
 }