fix: generic and runtime target type mappings with derived types

[justi7n]

Fix generic and runtime target type mappings with derived types

Description

If a user defined generic type or runtime target type mapping has a child mapping method which uses the [MapDerivedType] attribute, the parent method switch arm when clause was created wrongly (but only in that specific case). Fixes #1989

This got fixed in this PR

Fixes # (issue)

Checklist

• I did not use AI tools to generate this PR, or I have manually verified that the code is correct, optimal, and follows the project guidelines and architecture
• I understand that low-quality, AI-generated PRs will be closed immediately without further explanation
• The existing code style is followed
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• Performed a self-review of my code
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• The documentation is updated (as applicable)
• Unit tests are added/updated
• Integration tests are added/updated (as applicable, especially if feature/bug depends on roslyn or framework version in use)

[latonz]

I don't fully get this comment… either it is the runtime target type otherwise it is the runtime target type too?

[latonz]

Prefer pattern matching x is UserDefinedNewInstanceMethodMapping { IsDerivedTypeMapping: true }

[latonz]

This comment isn't 100% accurate anymore, is it?

[latonz]

This probably is a breaking change, isn't it? Document it in the breaking change docs.

[latonz]

Create a new test case reflecting the exact use-case from the referenced issue.

[latonz]

Add the exact same test but with [MapDerivedType(typeof(A), typeof(B))] instead of the generic one. I don't think it works as expected.

[latonz]

IMO the correct location to set this would be DerivedTypeMappingBuilder…

[justi7n]

Example for clarification:

Lets assume we have BananaDto : FruitDto, and FruitDto : OrganicDto. The previously released code in v4.3.1 would generate a code like

    public partial class DerivedTypeMapper
    {
        [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.3.1.0")]
        public partial T MapGeneric<T>(object source)
        {
            return source switch
            {
                // --- Relevant Part ---
                global::MapperlyRepro.Fruit x when typeof(T).IsAssignableFrom(typeof(global::MapperlyRepro.FruitDto)) => (T)(object)MapDerivedTypes(x),
                _ => throw new global::System.ArgumentException($"Cannot map {source.GetType()} to {typeof(T)} as there is no known type mapping", nameof(source)),
            };
        }

        [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.3.1.0")]
        public partial global::MapperlyRepro.FruitDto MapDerivedTypes(global::MapperlyRepro.Fruit source)
        {
            return source switch
            {
                global::MapperlyRepro.Banana x => MapToBananaDto(x),
                _ => throw new global::System.ArgumentException($"Cannot map {source.GetType()} to MapperlyRepro.FruitDto as there is no known derived type mapping", nameof(source)),
            };
        }

        [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.3.1.0")]
        private global::MapperlyRepro.BananaDto MapToBananaDto(global::MapperlyRepro.Banana source)
        {
               ...
        }
    }

by

[Mapper]
public partial class DerivedTypeMapper
{
    public partial T MapGeneric<T>(object source);

    [MapDerivedType<Banana, BananaDto>]
    public partial FruitDto MapDerivedTypes(Fruit source);
}

So previously, the mapping would work for:

• Banana => BananaDto.
• Banana => FruitDto,
• Banana => OrganicDto

If we reverse the AssignableFrom argument order (like currently in this PR), we would reverse the order of what would work:

• Banana => BananaDto
• Banana => FruitDto
• Banana => OrganicDto

So previously, if someone called derivedTypeMapper.MapGeneric<BananaDto>(new Banana()) he would get an exception. After this PR this works and someone gets an exception after calling derivedTypeMapper.MapGeneric<OrganicDto>(new Banana()) (because the underlying mapping method is for FruitDto, if it should work for OrganicDto it has to be defined as public partial OrganicDto MapDerivedTypes(Organic source)