Steps to reproduce:

1. Define a class with a method:

class foo
{
public:
    void bar();
};

1. Implement the bar method inside foo.cpp

void bar()
{
}

1. Modify the signature of bar inside foo.cpp

void bar(std::function<void (int)> func)
{
}

1. Press Alt + Return to modify the declaration of bar inside foo.h
2. Qt Creator will have left off the void return type, declaring a syntactically invalid function object.