[llvm-bugs] [Bug 44438] New: Some symbols are not built / undefined when used in decltype

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44438

            Bug ID: 44438
           Summary: Some symbols are not built / undefined when used in
                    decltype
           Product: clang
           Version: 9.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: julien at vivenot.fr
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Hi,

Actual results:

The following reduced test case fails to build A<C1>::foo. 
It results in a failed link :
  $ clang++ test.cpp
  /apps/dist/el6_gcc-9.2.0-64/bin/ld: /tmp/test-172f58.o:
(.rodata._ZTV1AI2C1E[_ZTV1AI2C1E]+0x10): undefined reference to `A<C1>::foo()'
  clang-9: error: linker command failed with exit code 1 (use -v to see
invocation)


Expected results:

Executable should build and link just fine, including this A<C1>::foo symbol
which is used here.


Build date and results :

I personally tested on :
clang version 9.0.1 (https://github.com/llvm/llvm-project.git
c1a0a213378a458fbea1a5c77b315c7dce08fd05)
Target: x86_64-unknown-linux-gnu
But I get similar results (missing A<C1>::foo) on godbolt for all versions
between 6.0.0 and trunk.


Additional information:

- Adding trailing return type information on 'Dispatcher::operator' return type
fixes the issue. Although it just repeats whats stated on the only return
instance. 
- Replacing 'dispatch' trailing return type with the actual type fixes the
issue. 


Test case:

#include <type_traits>

struct C1{};
struct C2{};
struct C3{};

struct IFace
{
  virtual void foo() = 0;
};

template <class T>
struct A : IFace
{
  void foo() override {};
};

template <template<typename> class F, typename Closure>
constexpr auto dispatch(int major, Closure fn)
// (*) the following does not build A<C1>::foo
  -> decltype(F<C1>{}(fn))
// (*) the following links (and builds A<C1>::foo)
//-> IFace*
{
  if (major == 1) return F<C1>{}(fn);
  if (major == 2) return F<C2>{}(fn);
  if (major == 3) return F<C3>{}(fn);
  return decltype(F<C1>{}(fn)){};
}

template <class Cn>
struct Dispatcher
{
  template<typename Closure>
  constexpr decltype(auto) operator()(Closure fn)
  // (*) adding the following also builds A<C1>::foo
  //  -> decltype(fn(Cn{}))
  {
    return fn(Cn{});
  }
};

int main()
{
  int i = 1;
  auto f = [](auto t) -> IFace*
      {
        return new A<decltype(t)>;
      };
  auto a = dispatch<Dispatcher>(i, f);
  return 1;
}

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20200102/3a600e7c/attachment.html>