Why can't I get compilation error of custom concept supplied with initializer list?

Question

Suppose I have 3 concepts:

• ostreamable
• istreamable
• iostreamable

Where the definitions:

template <typename T>
concept ostreamable = requires (std::ostream& os, T arg) {
    {os << arg} -> std::convertible_to<std::ostream&>;
};
template <typename T>
concept istreamable = requires (std::istream& is, T& arg) {
    {is >> arg} -> std::convertible_to<std::istream&>;
};
template <typename T>
concept iostreamable = ostreamable<T> && istreamable<T>;

Application:

iostreamable auto var1 = {4, 5, 10, 10}; // no error, but unexpected.
iostreamable auto var2 = 3.232; // no error, expected
iostreamable auto var3 = std::bitset<4>{0b1001}; // no error, expected
iostreamable auto var4 = std::vector{4, 10, 3, 10}; // compilation error, expected

When I use var1 with the requirements of said concept iostreamable such as applying with operator<< or operator>>, I expectedly got the verbose compilation error.

Answer 1

This appears to be a GCC bug.

Clang & MSVC reject your code.