MapValueAttribute does not support mapping non-nullable constant values, to nullable properties of the same type

[IanKemp]

Describe the bug
The following

enum TargetEnum
{
    Bar,
}

class Source
{
    string Quux { get; set; }
}

class Target
{
    string Quux { get; set; }
    TargetEnum? TheTarget { get; set; }
}

[MapValue(nameof(Target.TheTarget), TargetEnum.Bar))
static partial Target Map(this Source source);

incorrectly yields

RMG077: Cannot assign constant value TargetEnum.Bar of type TargetEnum to Target.TheTarget of type TargetEnum?

but should result in no error and valid generated code, because a non-null value of a type is always assignable to a nullable value of the same type.

The below cannot be used as a workaround, as typecasts are not permitted in attributes.

[MapValue(nameof(Target.TheTarget), (TargetEnum?)TargetEnum.Bar))

The below is a valid workaround, but undesirable.

[MapValue(nameof(Target.TheTarget), Use = nameof(GetNullableTargetEnumValue)))
static partial Target Map(this Source source);

static TargetEnum? GetNullableTargetEnumValue() => TargetEnum.Bar;

Environment (please complete the following information):

• Mapperly Version: 4.0.0
• Nullable reference types: enabled
• .NET Version: 8.0.403
• Target Framework: .net8.0
• Compiler Version: 4.11.0-3.24460.3 (5649376e)
• C# Language Version: 12.0
• IDE: VS2022.17.11.5
• OS: Windows 11 Pro 23H2 (22631.4169)

Additional context
#631

[latonz]

Thanks for reporting. Feel free to contribute 😉
Hint: Riok.Mapperly.Descriptors.MappingBodyBuilders.SourceValueBuilder.TryBuildConstantSourceValue

[latonz]

While reading the code I already fixed it: #1536

[latonz]

Fix released with 4.1.0-next.1, let me know if it works for you 😊

[IanKemp]

@latonz 4.1.0-next.1 fixes the case whereby

[MapValue(nameof(Target.TheTarget), TargetEnum.Bar))

raised RMG077, which is good... but that version now causes RMG078 to be raised for the previously-working

[MapValue(nameof(Target.TheTarget), Use = nameof(GetNullableTargetEnumValue)))
static partial Target Map(this Source source);

static TargetEnum? GetNullableTargetEnumValue() => TargetEnum.Bar;

which is incorrect.

[latonz]

@IanKemp My bad... #1538 should fix it definitely (hopefully).

[IanKemp]

@latonz thanks for your hard work - will wait for next .next version to drop and report back with verification.