[llvm-bugs] [Bug 25887] New: UBSan: missing check for accessing an inactive member of an union

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25887

            Bug ID: 25887
           Summary: UBSan: missing check for accessing an inactive member
                    of an union
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: gonzalobg88 at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Just discussed in this range-v3 issue:
https://github.com/ericniebler/range-v3/issues/239

both Eric and Casey give good examples. Here Eric's example:

template<typename F, typename S>
struct pair_data
{
    union
    {
        F non_const_first_;
        F const first;
    };
    union
    {
        S non_const_second_;
        S const second;
    };
};

template<typename F, typename S>
struct pair : private pair_data<F, S>
{
    using pair_data<F, S>::first;
    using pair_data<F, S>::second;
    pair() : pair_data<F, S>{} {}
    pair(F f, S s) : pair_data<F, S>{f, s} {}
    ~pair()
    {
        first.~F();
        second.~S();
    }
};

int main()
{
    pair<int, float> f{1, 3.14f};
    return f.first;  // UB
    // more UB: destructors of the non-active members (first, second) are
called
    // instead of the destructors of the active members (first_, second_)
}

Aggregate initialization of the union initialize the first member "first_".
Accessing first thus access a non-active member of the union and results in
undefined behavior.

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