Constness of object returned by QMap::value in Qt5 vs Qt6

[pptaszni]

I noticed that the code that doesn't compile with Qt5

#include <QMap>
 
class Foo {
public:
    void setX(int x) { _x = x; }
private:
    int _x;
};
 
int main() {
    QMap<int, Foo> m;
    m.value(1).setX(1);
}

#include <QMap>

class Foo {
public:
    void setX(int x) { _x = x; }
private:
    int _x;
};

int main() {
    QMap<int, Foo> m;
    m.value(1).setX(1);
}

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suddenly started to compile with Qt6. This is because of change in QMap::value signature from

const T QMap::value(const Key &key, const T &defaultValue = T()) const

const T QMap::value(const Key &key, const T &defaultValue = T()) const

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to

T QMap::value(const Key &key, const T &defaultValue = T()) const

T QMap::value(const Key &key, const T &defaultValue = T()) const

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What is the rationale behind this change? As you can see, the Qt5 version prevented developers from making accidental mistakes, if they assumed that `T` is returned by reference.

There are probably more changes like this, another method that I am aware of, with the same effect, is

const T QMap::operator[](const Key &key) const

const T QMap::operator[](const Key &key) const

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