[llvm-bugs] [Bug 45872] New: Unclear warning -Wreturn-std-move

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45872

            Bug ID: 45872
           Summary: Unclear warning -Wreturn-std-move
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jvapen at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

// https://godbolt.org/z/Fb7vvs
// clang++ -Wreturn-std-move -std=c++17 t.cpp
#include <vector>

auto f() -> std::vector<int>;

auto g(std::vector<int> &) -> void;

auto h() -> std::vector<int>
{
    auto v = f();
    g(v);
    return v;
}

auto i() -> std::vector<int>
{
    auto &&v = f();  // WARNING: local variable 'v' will be copied despite
being returned by name
    g(v);
    return v;
}

--------------------------

The above code gives a warning with return-std-move. However, if we replace
`auto &&` by `auto`, as done in h(), we don't get a warning.
In not familiar enough with the standard to know if this warning is indeed
doing the right thing.

If this is correct, it would be really helpful if the warning message contained
a note explaining what is the reason for this not being copied by name. This
way, the root cause could be tackled instead of the symptom.

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