fix: improve error messages for unresolved generic type arguments in test generation

TUnit.Core.SourceGenerator/Generators/TestMetadataGenerator.cs

@@ -321,12 +321,18 @@ private static void GenerateTestMetadata(CodeWriter writer, TestMethodMetadata t
                 .Any(a => a.AttributeClass?.Name == "MethodDataSourceAttribute" &&
                           InferClassTypesFromMethodDataSource(compilation, testMethod, a) != null);
 
-            if (hasTypedDataSource || hasGenerateGenericTest || testMethod.IsGenericMethod || hasClassArguments || hasTypedDataSourceForGenericType || hasMethodArgumentsForGenericType || hasMethodDataSourceForGenericType)
+            // Check for class-level data sources that could help resolve generic type arguments
+            var hasClassDataSources = testMethod.IsGenericType && testMethod.TypeSymbol.GetAttributesIncludingBaseTypes()
+                .Any(a => DataSourceAttributeHelper.IsDataSourceAttribute(a.AttributeClass));
+
+            if (hasTypedDataSource || hasGenerateGenericTest || testMethod.IsGenericMethod || hasClassArguments || hasTypedDataSourceForGenericType || hasMethodArgumentsForGenericType || hasMethodDataSourceForGenericType || hasClassDataSources)
             {
                 GenerateGenericTestWithConcreteTypes(writer, testMethod, className, uniqueClassName);
             }
             else
             {
+                // For generic classes with no way to resolve type arguments, this will generate
+                // GenericTestMetadata that the engine will fail with a clear error message
                 GenerateTestMetadataInstance(writer, testMethod, className, uniqueClassName);
             }
         }

TUnit.Engine/Building/TestBuilder.cs

@@ -397,7 +397,7 @@ await _objectLifecycleService.RegisterObjectAsync(
 
                                 if (metadata.TestClassType.IsGenericTypeDefinition && resolvedClassGenericArgs.Length == 0)
                                 {
-                                    throw new InvalidOperationException($"Cannot create instance of generic type {metadata.TestClassType.Name} with empty type arguments");
+                                    throw new InvalidOperationException($"Cannot create test for generic class '{metadata.TestClassType.Name}': No type arguments could be inferred. Add [GenerateGenericTest<ConcreteType>] to the class, or use a data source (like [ClassDataSource<T>] or [Arguments]) that provides constructor arguments to infer the generic type arguments from.");
                                 }
 
                                 var basicSkipReason = GetBasicSkipReason(metadata, attributes);
@@ -1727,7 +1727,7 @@ private Task<InstanceCreationResult> CreateInstanceForMethodDataSources(
 
             if (metadata.TestClassType.IsGenericTypeDefinition && resolvedClassGenericArgs.Length == 0)
             {
-                throw new InvalidOperationException($"Cannot create instance of generic type {metadata.TestClassType.Name} with empty type arguments");
+                throw new InvalidOperationException($"Cannot create test for generic class '{metadata.TestClassType.Name}': No type arguments could be inferred. Add [GenerateGenericTest<ConcreteType>] to the class, or use a data source (like [ClassDataSource<T>] or [Arguments]) that provides constructor arguments to infer the generic type arguments from.");
             }
 
             // Create instance factory

TUnit.Engine/Services/TestGenericTypeResolver.cs

@@ -97,7 +97,9 @@ private static Type[] ResolveClassGenericArguments(
             // For classes with parameterless constructors, throw a specific exception
             // that can be caught and handled by the caller
             throw new GenericTypeResolutionException(
-                $"Could not resolve type for generic parameter(s) of type '{genericClassType.Name}' from constructor arguments. Type inference from method data may be required.");
+                $"Cannot create test for generic class '{genericClassType.Name}': No type arguments could be inferred. " +
+                $"Add [GenerateGenericTest<ConcreteType>] to the class, or use a data source " +
+                $"(like [ClassDataSource<T>] or [Arguments]) that provides constructor arguments to infer the generic type arguments from.");
         }
 
         // Resolve all generic parameters
@@ -108,7 +110,8 @@ private static Type[] ResolveClassGenericArguments(
             if (!typeMapping.TryGetValue(genericParam, out var resolvedType))
             {
                 throw new GenericTypeResolutionException(
-                    $"Could not resolve type for generic parameter '{genericParam.Name}' of type '{genericClassType.Name}'");
+                    $"Cannot create test for generic class '{genericClassType.Name}': Could not resolve type for generic parameter '{genericParam.Name}'. " +
+                    $"Add [GenerateGenericTest<ConcreteType>] to the class, or use a data source that provides constructor arguments to infer the generic type arguments from.");
             }
             resolvedTypes[i] = resolvedType;
         }

TUnit.Example.Asp.Net.TestProject/TUnit.Example.Asp.Net.TestProject.csproj

@@ -4,6 +4,7 @@
 
     <PropertyGroup>
         <TargetFrameworks>net10.0</TargetFrameworks>
+        <InterceptorsNamespaces>$(InterceptorsNamespaces);Microsoft.AspNetCore.OpenApi.Generated</InterceptorsNamespaces>
     </PropertyGroup>
 
     <ItemGroup>