Fix clashing MakeUnique name coming from user code.

[mwl4]

Consider following code:

#include "RmlUi/Core.h"

// client code, global namespace

template < typename T, typename ...Args >
inline std::unique_ptr< T > MakeUnique( Args &&...args )
{
    return ( std::unique_ptr< T >( new T( ::std::forward< Args >( args )... ) ) );
}

struct Invader
{
    Rml::String name;
};

void Foo( Rml::Context *context )
{
    if( Rml::DataModelConstructor constructor = context->CreateDataModel( "invaders" ) )
    {
        if( auto invader_handle = constructor.RegisterStruct<Invader>() )
        {
            invader_handle.RegisterMember( "name", &Invader::name );
        }
        // .........
    }
}

Compilation throws error:

2>rmlui\include\RmlUi\Core\DataStructHandle.h(154): error C2668: 'Rml::MakeUnique': ambiguous call to overloaded function
2>rmlui\include\RmlUi\Core\../Config/Config.h(189): note: could be 'std::unique_ptr<Rml::MemberObjectDefinition<Object,MemberType>,std::default_delete<Rml::MemberObjectDefinition<Object,MemberType>>> Rml::MakeUnique<Rml::MemberObjectDefinition<Object,MemberType>,Rml::VariableDefinition*&,MemberTypeInvader::* &>(Rml::VariableDefinition *&,MemberType Invader::* &)'
2>        with
2>        [
2>            Object=Invader,
2>            MemberType=Rml::String
2>        ]
2>XXXXX(59): note: or       'std::unique_ptr<Rml::MemberObjectDefinition<Object,MemberType>,std::default_delete<Rml::MemberObjectDefinition<Object,MemberType>>> MakeUnique<Rml::MemberObjectDefinition<Object,MemberType>,Rml::VariableDefinition*&,MemberTypeInvader::* &>(Rml::VariableDefinition *&,MemberType Invader::* &)' [found using argument-dependent lookup]
2>        with
2>        [
2>            Object=Invader,
2>            MemberType=Rml::String
2>        ]

The reason behind this is ADL (https://en.cppreference.com/w/cpp/language/adl)

template<typename Object>
template<typename MemberType>
bool StructHandle<Object>::CreateMemberObjectDefinition(const String& name, MemberType Object::* member_ptr)
{
// ....................................
	struct_definition->AddMember(
		name,
		MakeUnique<MemberObjectDefinition<Object, MemberType>>(underlying_definition, member_ptr)
	);

Notice, that member_ptr is passed as function argument to MakeUnique. Since Object template parameter is defined within global namespace, then there are two matching MakeUnique functions. One from Rml's Config.h file, and the user's one.

This PR should resolve this issue by forcing certain namespace.

[mikke89]

Ah, thanks for the detailed PR. Makes sense to me, a subtle ADL strike.

[ShawnCZek]

This does sadly not fix all occurrences. There are more files with this problem:

• https://github.com/mikke89/RmlUi/blob/master/Include/RmlUi/Core/DataModelHandle.h
• https://github.com/mikke89/RmlUi/blob/master/Include/RmlUi/Core/DataTypeRegister.h

[mwl4]

@ShawnCZek I checked all MakeUniques along include header files and made fixes to them too.
#420

@mikke89 Review and merge it please.