Fix handling of nullable types in validations generator

Author: captainsafia

src/Http/Http.Extensions/gen/Microsoft.AspNetCore.Http.ValidationsGenerator/Extensions/ITypeSymbolExtensions.cs

@@ -46,8 +46,9 @@ public static ITypeSymbol UnwrapType(this ITypeSymbol type, INamedTypeSymbol enu
 
         if (type.NullableAnnotation == NullableAnnotation.Annotated)
         {
-            // Extract the underlying type from a reference type
-            type = type.OriginalDefinition;
+            // Remove the nullable annotation but keep any generic arguments, e.g. List<int>? → List<int>
+            // so we can retain them in future steps.
+            type = type.WithNullableAnnotation(NullableAnnotation.NotAnnotated);
         }
 
         if (type is INamedTypeSymbol namedType && namedType.IsEnumerable(enumerable) && namedType.TypeArguments.Length == 1)

src/Http/Http.Extensions/gen/Microsoft.AspNetCore.Http.ValidationsGenerator/Parsers/ValidationsGenerator.TypesParser.cs

@@ -29,13 +29,14 @@ internal ImmutableArray<ValidatableType> ExtractValidatableTypes(IInvocationOper
         List<ITypeSymbol> visitedTypes = [];
         foreach (var parameter in parameters)
         {
-            _ = TryExtractValidatableType(parameter.Type.UnwrapType(wellKnownTypes.Get(WellKnownTypeData.WellKnownType.System_Collections_IEnumerable)), wellKnownTypes, ref validatableTypes, ref visitedTypes);
+            _ = TryExtractValidatableType(parameter.Type, wellKnownTypes, ref validatableTypes, ref visitedTypes);
         }
         return [.. validatableTypes];
     }
 
-    internal bool TryExtractValidatableType(ITypeSymbol typeSymbol, WellKnownTypes wellKnownTypes, ref HashSet<ValidatableType> validatableTypes, ref List<ITypeSymbol> visitedTypes)
+    internal bool TryExtractValidatableType(ITypeSymbol incomingTypeSymbol, WellKnownTypes wellKnownTypes, ref HashSet<ValidatableType> validatableTypes, ref List<ITypeSymbol> visitedTypes)
     {
+        var typeSymbol = incomingTypeSymbol.UnwrapType(wellKnownTypes.Get(WellKnownTypeData.WellKnownType.System_Collections_IEnumerable));
         if (typeSymbol.SpecialType != SpecialType.None)
         {
             return false;
@@ -126,7 +127,7 @@ internal ImmutableArray<ValidatableProperty> ExtractValidatableMembers(ITypeSymb
                         // Check if the property's type is validatable, this resolves
                         // validatable types in the inheritance hierarchy
                         var hasValidatableType = TryExtractValidatableType(
-                            correspondingProperty.Type.UnwrapType(wellKnownTypes.Get(WellKnownTypeData.WellKnownType.System_Collections_IEnumerable)),
+                            correspondingProperty.Type,
                             wellKnownTypes,
                             ref validatableTypes,
                             ref visitedTypes);
@@ -153,7 +154,7 @@ internal ImmutableArray<ValidatableProperty> ExtractValidatableMembers(ITypeSymb
                 continue;
             }
 
-            var hasValidatableType = TryExtractValidatableType(member.Type.UnwrapType(wellKnownTypes.Get(WellKnownTypeData.WellKnownType.System_Collections_IEnumerable)), wellKnownTypes, ref validatableTypes, ref visitedTypes);
+            var hasValidatableType = TryExtractValidatableType(member.Type, wellKnownTypes, ref validatableTypes, ref visitedTypes);
             var attributes = ExtractValidationAttributes(member, wellKnownTypes, out var isRequired);
 
             // If the member has no validation attributes or validatable types and is not required, skip it.

src/Http/Http.Extensions/test/ValidationsGenerator/ValidationsGenerator.ComplexType.cs

@@ -45,7 +45,8 @@ public class ComplexType
 
     public SubTypeWithInheritance PropertyWithInheritance { get; set; } = new SubTypeWithInheritance("some-value", default);
 
-    public List<SubType> ListOfSubTypes { get; set; } = [];
+    // Nullable to validate https://github.com/dotnet/aspnetcore/issues/61737
+    public List<SubType>? ListOfSubTypes { get; set; } = [];
 
     [DerivedValidation(ErrorMessage = "Value must be an even number")]
     public int IntegerWithDerivedValidationAttribute { get; set; }