Chrome Version       : 53.0.2774.3

I tried making event_with_latency_info.h use extern templates and ran into a couple of compiler bugs. Found a decent workaround though.

Context: https://codereview.chromium.org/2091213002/
Goal: Encapsulate definitions for `CanCoalesceWith` and `CoalesceWith`

https://codereview.chromium.org/2091213002/#ps120001 should have worked, but the build fails on the component build in win_chromium_compile_dbg_ng with

[18246/18288] LINK content_unittests.exe
FAILED: content_unittests.exe 
E:/b/depot_tools/python276_bin/python.exe gyp-win-tool link-wrapper environment.x86 False link.exe /nologo /OUT:content_unittests.exe /PDB:content_unittests.exe.pdb @content_unittests.exe.rsp
event_with_latency_info_unittest.obj :error LNK2019: unresolved external symbol "__declspec(dllimport) public: __thiscall content::EventWithLatencyInfo<class blink::WebGestureEvent>::EventWithLatencyInfo<class blink::WebGestureEvent>(class content::EventWithLatencyInfo<class blink::WebGestureEvent> &&)" (__imp_??0?$EventWithLatencyInfo@VWebGestureEvent@blink@@@content@@QAE@$$QAV01@@Z) referenced in function "protected: class content::EventWithLatencyInfo<class blink::WebGestureEvent> __thiscall content::`anonymous namespace'::EventWithLatencyInfoTest::CreateGestureEvent(enum blink::WebInputEvent::Type,double)" (?CreateGestureEvent@EventWithLatencyInfoTest@?A0x58bef626@content@@IAE?AV?$EventWithLatencyInfo@VWebGestureEvent@blink@@@3@W4Type@WebInputEvent@blink@@N@Z)
event_with_latency_info_unittest.obj :error LNK2019: unresolved external symbol "__declspec(dllimport) public: __thiscall content::EventWithLatencyInfo<class blink::WebMouseWheelEvent>...
event_with_latency_info_unittest.obj :error LNK2019: unresolved external symbol "__declspec(dllimport) public: __thiscall content::EventWithLatencyInfo<class blink::WebMouseEvent>...
event_with_latency_info_unittest.obj :error LNK2019: unresolved external symbol "__declspec(dllimport) public: __thiscall content::EventWithLatencyInfo<class blink::WebTouchEvent>...
content_unittests.exe : fatalerror LNK1120: 4 unresolved externals

Note these are the *move* constructors. They're being used for NVRO in event_with_latency_info_unittest.cc which has code like

  MouseEventWithLatencyInfo CreateMouseEvent(WebInputEvent::Type type,
                                             double timestamp) {
    MouseEventWithLatencyInfo mouse;
    mouse.event.type = type;
    mouse.event.timeStampSeconds = timestamp;
    return mouse;
  }

I seem to recall the standards committee deciding that there would be no implicitly generated move constructors, so it's probably a bug that NVRO there tries to use it. But, we can resolve this by just declaring a move constructor (and the rest so they aren't hidden by the move constructor):

  EventWithLatencyInfo(const EventWithLatencyInfo& other) = default;
  EventWithLatencyInfo(EventWithLatencyInfo&& other) = default;
  EventWithLatencyInfo& operator=(const EventWithLatencyInfo& other) = default;


But! Although this fixed Windows, it caused other platforms to fail in https://codereview.chromium.org/2091213002/#ps220001

Errors like

[5056/5365] LINK ./components_unittests
FAILED: components_unittests 
TOOL_VERSION=1465060049 ../../build/toolchain/mac/linker_driver.py /b/build/slave/cache/cipd/goma/gomacc ../../third_party/llvm-build/Release+Asserts/bin/clang++  -stdlib=libc++ /* snip */
Undefined symbols for architecture x86_64:
  "content::EventWithLatencyInfo<blink::WebMouseEvent>::operator=(content::EventWithLatencyInfo<blink::WebMouseEvent> const&)", referenced from:
      content::InputRouterImpl::SendMouseEventImmediately(content::EventWithLatencyInfo<blink::WebMouseEvent> const&) in input_router_impl.o
      content::TouchpadTapSuppressionController::ShouldDeferMouseDown(content::EventWithLatencyInfo<blink::WebMouseEvent> const&) in touchpad_tap_suppression_controller.o
  "content::EventWithLatencyInfo<blink::WebTouchEvent>::operator=(content::EventWithLatencyInfo<blink::WebTouchEvent> const&)", referenced from:
      content::TouchEventQueue::ForwardNextEventToRenderer() in touch_event_queue.o
      content::TouchEventQueue::TouchTimeoutHandler::StartIfNecessary(content::EventWithLatencyInfo<blink::WebTouchEvent> const&) in touch_event_queue.o
  "content::EventWithLatencyInfo<blink::WebGestureEvent>::operator=(content::EventWithLatencyInfo<blink::WebGestureEvent> const&)", referenced from:
      content::GestureEventQueue::QueueScrollOrPinchAndForwardIfNecessary(content::EventWithLatencyInfo<blink::WebGestureEvent> const&) in gesture_event_queue.o
      content::GestureEventQueue::ProcessGestureAck(content::InputEventAckState, blink::WebInputEvent::Type, ui::LatencyInfo const&) in gesture_event_queue.o
      std::__1::/* snip */

The bug here seems to be that the explicit instantiation of an extern template doesn't generate any code for `= default` methods. There's a gcc bug filed already here - https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51629 . Couldn't find a clang bug for it. But since only Windows needs the `= default` thing they can just be put behind guards.

A basic test case goes like:

$ head -n100 e.h e.cc extern_default.cc build.sh
==> e.h <==
class I {
 public:
  I() {}
  I& operator=(const I&);
};

template <class T>
class E {
 public:
  T t;
  E() : t() {}
  E& operator=(const E&) = default;
};

extern template class E<I>;

==> e.cc <==
#include "e.h"

I& I::operator=(const I& i) { return *this; }

template class E<I>;

==> extern_default.cc <==
#include "e.h"

int main() {
  E<I> e1, e2;
  e1 = e2;
}

==> build.sh <==
g++ extern_default.cc e.cc -Wall -std=c++11

$ bash build.sh
Undefined symbols for architecture x86_64:
  "E<I>::operator=(E<I> const&)", referenced from:
      _main in extern_default-66fb4a.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

$ nm e.o | c++filt
0000000000000060 S E<I>::E()
0000000000000020 S E<I>::E()
0000000000000040 S I::I()
0000000000000080 S I::I()
0000000000000000 T I::operator=(I const&)


Filing this to track, and cc some people who might care about it (or see an alternative diagnosis). Still need to hunt down (or file) a clang version of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51629, and put together a test case for MSVC.
 

Deleted: extern_default.tar.gz 421 bytes

extern_default.tar.gz 421 bytes Download