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array-member-variable construction using move constructor in GCC <= 9.3 but GCC > 9.3

March 23, 2023

With GCC++ <= 9.3 the code below has compile error about using move constructor which is being deleted:

<source>: In constructor 'Widget::Widget()':
<source>:16:24: error: use of deleted function 'Child::Child(Child&&)'
   16 |     Widget() : m_array{} {}
      |                        ^
<source>:11:5: note: declared here
   11 |     Child(Child&&) = delete;
      |     ^~~~~

With GCC++ >9.3 the error won’t be seen anymore.
Is it something about compiler specific issue or the code itself ?
As I comment in the code if the destructor is commented out the error won’t be seen.

https://godbolt.org/z/4s3o1sxPs

class Base {
public:
    Base() {}
    virtual ~Base() = default;//comment out error won't bee seen anymore
    Base(Base&&) = delete;
};

class Child  : public Base{
public:
    Child() = default;
    Child(Child&&) = delete;
};

class Widget {
public:
    Widget() : m_array{} {}
private:
    Child m_array[2];
};

int main()
{
    Widget wd{};
}

>Solution :

Given that the behavior changed between gcc 9.3 and 9.4; this looks like a compiler bug that got fixed.

In fact, looking at the list of C++ compiler bugs fixed in gcc 9.4, it’s not hard to find https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63707 "Brace initialization of array sometimes fails if no copy constructor" which seems to be the bug you encountered.