Use 404 vs 400 when versioning only by URL and no requested versions. F…

…ixes #911

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

@@ -11,6 +11,7 @@ namespace Asp.Versioning.Routing;
 internal sealed class ApiVersionPolicyJumpTable : PolicyJumpTable
 {
  private readonly bool versionsByUrl;
+ private readonly bool versionsByUrlOnly;
  private readonly bool versionsByMediaTypeOnly;
  private readonly RouteDestination rejection;
  private readonly IReadOnlyDictionary<ApiVersion, int> destinations;
@@ -32,6 +33,7 @@ internal sealed class ApiVersionPolicyJumpTable : PolicyJumpTable
  this.parser = parser;
  this.options = options;
  versionsByUrl = routePatterns.Count > 0;
+ versionsByUrlOnly = source.VersionsByUrl( allowMultipleLocations: false );
  versionsByMediaTypeOnly = source.VersionsByMediaType( allowMultipleLocations: false );
  }
 
@@ -61,15 +63,18 @@ public override int GetDestination( HttpContext httpContext )
  return destination;
  }
 
- // 2. short-circuit if a default version cannot be assumed
- if ( !options.AssumeDefaultVersionWhenUnspecified )
+ // 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 )
  {
- return rejection.Unspecified; // 400
+ return rejection.AssumeDefault;
  }
 
- // 3. 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
- return rejection.AssumeDefault;
+ // 3. unspecified
+ return versionsByUrlOnly
+ /* 404 */ ? rejection.Exit
+ /* 400 */ : rejection.Unspecified;
+
  case 1:
  rawApiVersion = apiVersions[0];