CS8601 while mapping properties annotated with `MaybeNullAttribute`

[hugoqribeiro]

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

Generated code produces compilation error CS8601 Possible null reference assignment when:

• Using nullable reference types
• Using [MaybeNull] attribute

Any option to have the mapper consider the MaybeNull attribute in the source property?

Declaration code

public partial class OriginalType
{
    [MaybeNull]
    public string Name { get; set; } = default!;
}

public partial class MappedType
{
    [MaybeNull]
    public string Name { get; set; }
}

[Mapper]
internal partial class OneMapper
{
    public partial MappedType Map(OriginalType source);
}

Actual relevant generated code

internal partial class OneMapper
{
    [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.3.1.0")]
    public partial global::MapperlyBug.MappedType Map(global::MapperlyBug.OriginalType source)
    {
        var target = new global::MapperlyBug.MappedType();
        target.Name = source.Name;
        return target;
    }
}

Expected relevant generated code

internal partial class OneMapper
{
    [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.3.1.0")]
    public partial global::MapperlyBug.MappedType Map(global::MapperlyBug.OriginalType source)
    {
        var target = new global::MapperlyBug.MappedType();
        target.Name = source.Name!;
        // or
        //if (source.Name != null)
        //{
        //    target.Name = source.Name;
        //}
        return target;
    }
}

Environment (please complete the following information):

• Mapperly Version: 4.3.1
• Nullable reference types: enabled
• .NET Version: 10.0.201
• Target Framework: net10.0
• C# Language Version: 14.0
• IDE: VS 2026

Additional context

Changing the original code to use required properties is not an option because it would change the semantics of a large existing code base (forcing setting a bunch of properties at object initialization).
Additionally, most of the real classes are used as REST payloads (JSON serialization) and are also annotated with the [Required] attribute.
99,9% of the times using the null forgiving operator would be fine because the source of the mapping has already been validated.

[hugoqribeiro]

Hi @latonz.

I noticed that release 5.0.0-next.3 mentioned this issue. So I tried to use it but there is something wrong because the problem persists.

In case it's helpful, I have a repo that reproduces it:

https://github.com/hugoqribeiro/issues
https://github.com/hugoqribeiro/issues/tree/main/src/2026.03.Riok.Mapperly

[latonz]

Thank you for reporting, I'll look into it again.