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From: "gccbugbjorn at fahller dot se" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/100047] New: False -Wmaybe-uninitialized on one var
 depending on type of other var
Date: Mon, 12 Apr 2021 13:18:49 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100047

            Bug ID: 100047
           Summary: False -Wmaybe-uninitialized on one var depending on
                    type of other var
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gccbugbjorn at fahller dot se
  Target Milestone: ---

With the following C++17 code, there's a warning that m_size can be used
uninitialized if m_ptr is a std::unique_ptr<>, but not if it is a raw point=
er.
This only happens when some optimization is enabled. -Og is enough. The bug
also applies to gcc-10, gcc-9 and gcc-8 (have not checked older versions).

#include <cstdint>
#include <cstdlib>
#include <optional>
#include <memory>
#include <string>

#if EXPOSE_BUG
using buffer_ptr =3D std::unique_ptr<uint8_t>;
#else
using buffer_ptr =3D const uint8_t*;
#endif

const uint8_t* get(const std::unique_ptr<uint8_t>& ptr)
{
    return ptr.get();
}
const uint8_t* get(const uint8_t* ptr)
{
    return ptr;
}

class packet_buffer
{
public:
    using const_iterator =3D const uint8_t*;
    packet_buffer() : m_ptr(nullptr), m_size(0) {}
    packet_buffer(buffer_ptr ptr, uint16_t size)
        : m_ptr(std::move(ptr)), m_size(size)
    {
    }
    size_t size() const { return m_size; }           // <- here
    const uint8_t* data() const { return get(m_ptr); }
    const_iterator begin() const { return data(); }
    const_iterator end() const { return data() + size(); }
private:
    buffer_ptr m_ptr;
    uint16_t m_size =3D 0;
};


std::string func()
{
    auto f =3D
        [](std::optional<packet_buffer> result) {
          auto&& b =3D result.value();
          return std::string(b.begin(), b.end());
        };
    return f(std::nullopt);
}

Godbolt link:
https://godbolt.org/z/63z8qzcxv=