Fix Assuming Default Version with Version-Neutral

src/AspNetCore/WebApi/src/Asp.Versioning.Http/Routing/ApiVersionPolicyJumpTable.cs

@@ -63,17 +63,18 @@ public override int GetDestination( HttpContext httpContext )
         switch ( apiVersions.Count )
         {
             case 0:
-                // 1. version-neutral endpoints take precedence
-                if ( destinations.TryGetValue( ApiVersion.Neutral, out destination ) )
+                // 1. IApiVersionSelector cannot be used yet because there are no candidates that an
+                //    aggregated version model can be computed from to select the default API version.
+                //    version-neutral endpoints are still included in these candidates
+                if ( options.AssumeDefaultVersionWhenUnspecified )
                 {
-                    return destination;
+                    return rejection.AssumeDefault;
                 }
 
-                // 2. IApiVersionSelector cannot be used yet because there are no candidates that an
-                //    aggregated version model can be computed from to select the 'default' API version
-                if ( options.AssumeDefaultVersionWhenUnspecified )
+                // 2. use version-neutral endpoints, if any
+                if ( destinations.TryGetValue( ApiVersion.Neutral, out destination ) )
                 {
-                    return rejection.AssumeDefault;
+                    return destination;
                 }
 
                 httpContext.Features.Set( policyFeature );

src/AspNetCore/WebApi/src/Asp.Versioning.Http/Routing/EdgeBuilder.cs

@@ -57,12 +57,18 @@ public void Add( RouteEndpoint endpoint )
 
     public void Add( RouteEndpoint endpoint, ApiVersion apiVersion, ApiVersionMetadata metadata )
     {
-        // use a singleton of all route patterns that version by url segment. this
-        // is needed to extract the value for selecting a destination in the jump
-        // table. any matching template will do and every edge should have the
-        // same list known through the application, which may be zero
+        // use a singleton of all route patterns that version by url segment. this is needed to extract the value for
+        // selecting a destination in the jump table. any matching template will do and every edge should have the same
+        // list known through the application, which may be zero
         var key = new EdgeKey( apiVersion, metadata, routePatterns );
+
         Add( ref key, endpoint );
+
+        // include version-neutral endpoints when assuming the default so they are also considered when unspecified
+        if ( unspecifiedAllowed && metadata.IsApiVersionNeutral && apiVersion == ApiVersion.Neutral )
+        {
+            Add( ref assumeDefault, endpoint );
+        }
     }
 
     private void Add( ref EdgeKey key, RouteEndpoint endpoint )