Building Mixxx 2.4 fails with Xcode 12.4

[daschuer]

Bug Description

It looks like it insists to use the copy constructor when using operator[]

../src/effects/backends/effectprocessor.h:204:37: note: in instantiation of member function 'ChannelHandleMap<unique_ptr_vector<BalanceGroupState> >::operator[]' requested here
        auto& outputChannelStates = m_channelStateMatrix[inputChannel];
                                    ^
../src/effects/backends/builtin/balanceeffect.cpp:87:21: note: in instantiation of member function 'EffectProcessorImpl<BalanceGroupState>::initializeInputChannel' requested here
void BalanceEffect::loadEngineEffectParameters(
                    ^
/Applications/Xcode_12.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:2518:3: note: copy constructor is implicitly deleted because 'unique_ptr<BalanceGroupState, std::__1::default_delete<BalanceGroupState> >' has a user-declared move constructor
  unique_ptr(unique_ptr&& __u) _NOEXCEPT
  ^
1 error generated.

Version

2.4-beta

OS

macOS

[Swiftb0y]

fixed by #11765