else if ((allowedRequiredModifierType & AllowedRequiredModifierType.System_Runtime_CompilerServices_OutAttribute) != 0 &&
modifierType.IsWellKnownTypeOutAttribute())
{
current = AllowedRequiredModifierType.System_Runtime_CompilerServices_OutAttribute;
}
internal static bool IsWellKnownTypeOutAttribute(this TypeSymbol typeSymbol) => typeSymbol.IsWellKnownInteropServicesTopLevelType("OutAttribute");
It loos like the type is actually in InteropServices namespace rather than in CompilerServices.
This doesn't affect correctness of the behavior, I think.