[llvm-bugs] [Bug 37089] New: clang does not generate a template instance in a very specific case

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37089

            Bug ID: 37089
           Summary: clang does not generate a template instance in a very
                    specific case
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++14
          Assignee: unassignedclangbugs at nondot.org
          Reporter: blastrock at free.fr
                CC: llvm-bugs at lists.llvm.org

I have a strange bug where clang seems to forget to generate a template
instance.

Here is the code, reduced as much as I could:

    template <class T>
    T declval()
    {
      T* v = nullptr;
      return *v;
    }
    template <class F, class... Args>
    auto do_call(F func, Args&&... args) -> decltype(func(args...))
    {
      return func(args...);
    }

    template <class F, class... Args>
    struct __func
    {
      virtual void buggy_method()
      {
        do_call(declval<F>(), Args{}...);
      }
    };

    template <typename C>
    void async_resumable(C cb)
    {
      auto l = [cb](auto&&... args) { cb(args...); };
      __func<decltype(l), int> y;
    }

    int main()
    {
      async_resumable([&](auto) -> void { async_resumable([](int) {}); });
    }

On clang 5, 6 and trunk, compiling this gives the following error:

    $ /usr/bin/clang++-5.0  -std=c++14 -o /dev/null ok.ii 
    /tmp/ok-5eaa37.o:(.rodata+0x80): undefined reference to
`_ZN6__funcIZ15async_resumableIZZ4mainENK3$_0clIiEEvT_EUliE_EvS3_EUlDpOT_E_JiEE12buggy_methodEv'
    clang: error: linker command failed with exit code 1 (use -v to see
invocation)

- If you remove the "-> void" in the last lambda, the symbol is generated and
the code compiles
- If you replace the "auto" argument in the last lambda by "int", it also works
- GCC 7 compiles the code successfully in all cases.

If it's of any matter, in real life, that __func class is the one in
std::function in libc++.

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