polymorphic mappings via [MapDerivedType] on child [UseMapper] classes fail

[ChaseFlorell]

Please do the checklist before filing an issue:

• I have read the documentation, including the FAQ
• I can reproduce the bug using the latest prerelease version
• I have searched existing discussion and issue to avoid duplicates

Describe the bug
Given a child mapping class using the [UseMapper] attribute
When the child mapping exposes a polymorphic mapping as defined with a [MapDerivedType]
Then the parent mapper should use IsAssignableTo instead of IsAssignableFrom

Repro
A link to a repro Gist or GitHub repository or a link to a Mapperly fork with a failing test.

Actual relevant generated code

// Actual relevant code generated by Mapperly
[global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.2.1.0")]
public partial T Map<T>(object source)
{
    return source switch
    {
        global::MapperlyRepro.FooBaseDto x when typeof(T).IsAssignableFrom(typeof(global::MapperlyRepro.FooBase)) => (T)(object)_childMapper.Map(x),
        _ => throw new global::System.ArgumentException($"Cannot map {source.GetType()} to {typeof(T)} as there is no known type mapping", nameof(source)),
    };
}

Expected relevant generated code

// The generated code how you expect it to look like
[global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.2.1.0")]
public partial T Map<T>(object source)
{
    return source switch
    {
        global::MapperlyRepro.FooBaseDto x when typeof(T).IsAssignableTo(typeof(global::MapperlyRepro.FooBase)) => (T)(object)_childMapper.Map(x),
        _ => throw new global::System.ArgumentException($"Cannot map {source.GetType()} to {typeof(T)} as there is no known type mapping", nameof(source)),
    };
}

Environment (please complete the following information):

• Mapperly Version: 4.3.0-next.6
• Nullable reference types: Enabled
• .NET Version: 9.0.305
• Target Framework: net9.0
• Compiler Version: 4.14.0-3.25413.5
• C# Language Version: 13.0
• IDE: Rider 2025.1.7
• OS: macOS 15.5 (24F74)

Additional context
I think that when the Parent mapper is generating the switch statement for a child mapper, and the target method is decorated with the [MapDerivedType] attribute, then we need to use the IsAssignableTo method in order to support polymorphism.

[ChaseFlorell]

note: the current workaround is to manually configure each method that has a derived type mapping.
unfortunately this becomes cumbersome if there are many many child mappers with polymorphic maps.

[Mapper]
public sealed partial class Mapper
{
    public partial T Map<T>(object source) => (T) Map(source, typeof(T));

    public object Map(object sender, Type targetType) =>
        source switch
            {
                FooBaseDto x => _childMapper.Map(x); // this is yucky
                _ => MapInternal(sender, targetType);
            }
    
    private partial object MapInternal(object sender, Type targetType);

    [UseMapper] private readonly ChildMapper _childMapper = new();
}

[Mapper]
internal sealed partial class ChildMapper
{
    [MapDerivedType<FooDto, Foo>]
    public partial FooBase Map(FooBaseDto source);
}

[justi7n]

I'll take a look at it.

[justi7n]

My findings related to this issue are the following:

Lets assume we have the mapping methods:

• runtimeTargetTypeMapper.Map(banana, typeof(BananaDto));
• runtimeTargetTypeMapper.Map(banana, typeof(FruitDto));
• derivedTypeMapper.Map<BananaDto>(banana);
• derivedTypeMapper.Map<FruitDto>(banana);

// Runtime target type mapper
public partial object Map(object source, Type targetType);
private partial BananaDto MapToBananaDto(Banana source);

// Derived type mapper
public partial T Map<T>(object source);
[MapDerivedType<Banana, BananaDto>]
public partial FruitDto MapFruit(Fruit source);

These 2 cases should always work:

• Banana => BananaDto
• Banana => FruitDto

Mappings:

• Runtime target type parameter uses the extended class as a switch type case Banana x when targetType.IsAssignableFrom(typeof(BananaDto)) => OK
• Derived types with generics uses the base class as a switch type case Fruit x when typeof(T).IsAssignableFrom(typeof(FruitDto)) => X
• Mixed runtime target type and derived types switch type case Fruit x when targetType.IsAssignableFrom(typeof(FruitDto)) => X

As soon as the Base class is on the right side of the IsAssignableFrom statement the current approach doesnt work. This is the case as soon as derived types are involved, combined with generics or runtime target type mappings.

Fruit when typeof(T).IsAssignableFrom(typeof(FruitDto)) should be Fruit when typeof(FruitDto).IsAssignableFrom(typeof(T))/Fruit when typeof(T).IsAssignableTo(typeof(FruitDto)).

I'll implement a fix for that