[llvm-bugs] [Bug 50346] New: Implicitly-defined default constructor not generated for nested class

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50346

            Bug ID: 50346
           Summary: Implicitly-defined default constructor not generated
                    for nested class
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: alex.franchuk at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Code sample:

#include <optional>

struct Outer {
    struct Inner {
        int a = 0;
    };
    std::optional<Inner> inner;
};

int main() {
    Outer o;
    o.inner.emplace();
    return 0;
}

https://godbolt.org/z/jKoG9a7e4


In the above, the `emplace()` call causes an error, stating that the class
(`Outer::Inner`) does not have a constructor that matches the arguments. Note
that this is specifically an issue in combination with libstdc++'s impl of
std::optional; libc++ (since clang 9) seems to work (though prior to clang 9 it
generates the same error).

If you do `std::optional<Outer::Inner>`, it also fails (though it will succeed
if you remove the `inner` member).

If you change `int a = 0` to `int a` (without the default value), it will
succeed.

If you add an `Inner() = default`, it still fails, however adding `Inner() {}`
will make it succeed.

This may be loosely related to bug 15886.

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