[LLVMbugs] [Bug 13060] New: Need warning for returning a C++11 rvalue reference to a local

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13060

             Bug #: 13060
           Summary: Need warning for returning a C++11 rvalue reference to
                    a local
           Product: clang
           Version: trunk
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: arthur.j.odwyer at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


(I've stolen the formatting from Bug 12566, but this is a different issue.)

If I understand the semantics of rvalue references correctly, then the
following code should produce a warning for returning a reference to the local
s in foo():

std::string &&foo() {
    std::string s = "hello";
    return std::move(s);
}

similar to the warning given when returning an lvalue reference to a local:

warning: reference to stack memory associated with local variable
      'n' returned [-Wreturn-stack-address]
    return &n;


The reason is that the return value --- *if* I understand rvalue references
correctly --- is going to be an xvalue to a variable ("s") that has already
expired. Even if the caller immediately moves the result of foo() into a new
location using a move-constructor or move-assignment operator, it's too late.
It's too late as soon as we leave the scope of foo() and local variable "s" is
destructed.

Am I wrong about the semantics of the rvalue reference in this code? Am I right
but there is a good technical reason *not* to give a warning? Or is this a good
warning to give, but nobody's gotten around to it yet?


Incidentally, I believe that returning an rvalue reference to a reference
*parameter* is hunky-dory.

std::string &&okay(std::string &&trashme) {
    trashme = "hello";
    return trashme;  // This should NOT give the same warning,
    // although it is allegedly still dangerous.
    // See http://stackoverflow.com/questions/6006527
}

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