[Bug c++/95269] New: Lambda is allowed to capture any constexpr variable without specifying any captures

["dragondreamer at live dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95269

            Bug ID: 95269
           Summary: Lambda is allowed to capture any constexpr variable
                    without specifying any captures
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dragondreamer at live dot com
  Target Milestone: ---

G++ (at least these versions: 8.2.1, 9.2.1, 10.1.0, trunk) successfully
compiles the following code:

// ===================================
template<typename Lambda>
constexpr void test(Lambda lambda)
{
    constexpr auto result = lambda();
    static_assert(result.value == 1);
}

struct teststruct
{
    int value = 0;
};

constexpr auto get_value()
{
    teststruct result{};
    result.value = 1;
    return result;
}

int main()
{
    constexpr auto value = get_value();
    //Here's a lambda without any captures
    auto options_lambda = []() { return value; };
    test(options_lambda);
}
// ===================================

This code instead should produce an error when trying to create a lambda which
uses "value" without capturing it. This is the exact behavior of Clang and
MSVC, which both produce an error message.

As far as I understand, the following rule must be applied here:
"If a lambda expression (or an instantiation of a generic lambda's function
call operator) ODR-uses this or any variable with automatic storage duration,
it must be captured by the lambda expression."