[clang++] Clang fails to call initializer_list constructor in specific cases

[davidkjaer]

There seems to be a very specific case where clang does not call an initializer_list constructor even though it should apply.
The issues arises when the class Foo has an std::initializer_list<Foo> constructor and you create an object like thisFoo f{Foo{}};:

#include <initializer_list>
#include <iostream>
#include <vector>

class Foo
{
public:
    Foo()
    {
    }

    explicit Foo(std::initializer_list<Foo> l)
    {
        std::cout << "Initializer list constructor\n";
    }

    void hi()
    {
        std::cout << "Hello\n";
    }
};

int main() {
    Foo other;
    Foo f{std::move(other)};
    f.hi();
}

When compiled with clang 18.1 the above example prints:
Hello

On both gcc and msvc the example prints:
Initializer list constructor
Hello

If I switch the initialization to Foo f{{std::move(other)}} or Foo f{Foo{}, Foo{}} clang correctly calls the initializer_list constructor.

[efriedma-quic]

Could you explain why you think gcc is right here? I'm not an expert on overload resolution, but at first glance, Foo(Foo&&) should rank higher than Foo(std::initializer_list<Foo>) given the extra conversion required to construct the std::initializer_list.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (davidkjaer)

There seems to be a very specific case where clang does not call an initializer_list constructor even though it should apply. The issues arises when the class `Foo` has an `std::initializer_list<Foo>` constructor and you create an object like this`Foo f{Foo{}};`:

#include <initializer_list>
#include <iostream>
#include <vector>

class Foo
{
public:
    Foo()
    {
    }

    explicit Foo(std::initializer_list<Foo> l)
    {
        std::cout << "Initializer list constructor\n";
    }

    void hi()
    {
        std::cout << "Hello\n";
    }
};

int main() {
    Foo other;
    Foo f{std::move(other)};
    f.hi();
}

When compiled with clang 18.1 the above example prints:
Hello

On both gcc and msvc the example prints:
Initializer list constructor
Hello

If I switch the initialization to Foo f{{std::move(other)}} or Foo f{Foo{}, Foo{}} clang correctly calls the initializer_list constructor.

[efriedma-quic]

Actually, digging a bit more, this might be a duplicate of #24186 ?

[davidkjaer]

Yes. That definitely looks like a duplicate.

I have deleted my previous comment since I had missed the word "aggregate" making my conclusion incorrect. No need to leave misinformation on the internet :)