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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Implicitly-defined default constructor not generated for nested class"
href="https://bugs.llvm.org/show_bug.cgi?id=50346">50346</a>
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<th>Summary</th>
<td>Implicitly-defined default constructor not generated for nested class
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>alex.franchuk@gmail.com
</td>
</tr>
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>Code sample:
#include <optional>
struct Outer {
struct Inner {
int a = 0;
};
std::optional<Inner> inner;
};
int main() {
Outer o;
o.inner.emplace();
return 0;
}
<a href="https://godbolt.org/z/jKoG9a7e4">https://godbolt.org/z/jKoG9a7e4</a>
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 <a class="bz_bug_link
bz_status_NEW "
title="NEW - defaulted default constructor of class cannot be used by non-static data member initializer which appears before end of class definition"
href="show_bug.cgi?id=15886">bug 15886</a>.</pre>
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