CompileError

In file included from /usr/include/c++/7.3.1/memory:80:0,
                 from test/../../../test/MocCompilationIssue.h:4,
                 from test/moc_MocCompilationIssue.cpp:9:
/usr/include/c++/7.3.1/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = MyClassB]’:
/usr/include/c++/7.3.1/bits/unique_ptr.h:268:17:   required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = MyClassB; _Dp = std::default_delete<MyClassB>]’
test/../../../test/MocCompilationIssue.h:22:40:   required from here
/usr/include/c++/7.3.1/bits/unique_ptr.h:76:22: error: invalid application of ‘sizeof’ to incomplete type ‘MyClassB’
  static_assert(sizeof(_Tp)>0,
                      ^

MocCompilationIssue.cpp

#include "MocCompilationIssue.h"

class MyClassA {};
class MyClassB {};

MocCompiles::~MocCompiles() = default;

MocDoesntCompiles::~MocDoesntCompiles() = default;

MocCompilationIssue.h

#pragma once

#include <QObject>
#include <memory>


class MyClassA;

class MocCompiles : public QObject
{
    Q_OBJECT
    std::unique_ptr<MyClassA> member;
public:
    ~MocCompiles();
};


class MyClassB;
class MocDoesntCompiles : public QObject
{
    Q_OBJECT
    std::unique_ptr<MyClassB> member{}; // <-- Note curly braces
public:
    ~MocDoesntCompiles();
};