Return invalid_request when no response_type is provided

src/IdentityServer/Validation/Default/AuthorizeRequestValidator.cs

@@ -392,7 +392,7 @@ private AuthorizeRequestValidationResult ValidateCoreParameters(ValidatedAuthori
         if (responseType.IsMissing())
         {
             LogError("Missing response_type", request);
-            return Invalid(request, OidcConstants.AuthorizeErrors.UnsupportedResponseType, "Missing response_type");
+            return Invalid(request, OidcConstants.AuthorizeErrors.InvalidRequest, "Missing response_type");
         }
 
         // The responseType may come in in an unconventional order.

test/IdentityServer.IntegrationTests/Conformance/Basic/ResponseTypeResponseModeTests.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Duende Software. All rights reserved.
+// Copyright (c) Duende Software. All rights reserved.
 // See LICENSE in the project root for license information.
 
 
@@ -10,6 +10,7 @@
 using Duende.IdentityServer.Models;
 using Duende.IdentityServer.Test;
 using FluentAssertions;
+using IdentityModel;
 using IntegrationTests.Common;
 using Xunit;
 
@@ -111,6 +112,6 @@ public async Task Request_missing_response_type_rejected()
         _mockPipeline.BrowserClient.AllowAutoRedirect = true;
         var response = await _mockPipeline.BrowserClient.GetAsync(url);
 
-        _mockPipeline.ErrorMessage.Error.Should().Be("unsupported_response_type");
+        _mockPipeline.ErrorMessage.Error.Should().Be(OidcConstants.AuthorizeErrors.InvalidRequest);
     }
 }

test/IdentityServer.UnitTests/Validation/AuthorizeRequest Validation/Authorize_ProtocolValidation_Invalid.cs

@@ -206,7 +206,7 @@ public async Task Missing_ResponseType()
         var result = await validator.ValidateAsync(parameters);
 
         result.IsError.Should().BeTrue();
-        result.Error.Should().Be(OidcConstants.AuthorizeErrors.UnsupportedResponseType);
+        result.Error.Should().Be(OidcConstants.AuthorizeErrors.InvalidRequest);
     }
 
     [Fact]