[llvm-bugs] [Bug 41576] New: Bogus "reference to local variable error" with lambda in fold expression over comma and assignment

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41576

            Bug ID: 41576
           Summary: Bogus "reference to local variable error" with lambda
                    in fold expression over comma and assignment
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++'17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: vittorio.romeo at outlook.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

The following code

    #include <tuple>

    template <class T>
    struct X
    {
        template <class F>
        void f(F f)
        {
            f(0);
        }
    };

    template <class... Xs>
    void bug(X<Xs>... xs)
    {
        std::tuple<Xs...> t;

        std::apply([&](auto&... ys)
        {   
            (xs.f([&](auto y)
            {
                ys = y;
            }), ...);
        }, t);
    }

    int main()
    {
        bug(X<int>{});
    } 

produces a bogus error with clang++ (version 9.0.0 (trunk 359058))

    <source>:20:10: error: reference to local variable 'xs' declared in
enclosing function 'bug<int>'
            (xs.f([&](auto y)
            ^

The bug can be reproduced on godbolt.org here:
https://gcc.godbolt.org/z/NNLI5p

---

Changing the lambda to the following

    std::apply([&xs...](auto&... ys)
    {   
        (xs.f([&ys...](auto y)
        {
            ys = y;
        }), ...);
    }, t);

fixes the error:
https://gcc.godbolt.org/z/hp2hMN

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