Make base::Optional constructor constexpr.

Make base::Optional constructor constexpr.

BUG=

Committed: https://crrev.com/9494e454fa120c3d77f87c66142ef35f85ff225b
Cr-Commit-Position: refs/heads/master@{#427311}

[alshabalin, 2016-10-21 09:15:34]

Description was changed from

==========
Make base::Optional constructor constexpr.

BUG=
==========

to

==========
Make base::Optional constructor constexpr.

BUG=
==========

[alshabalin, 2016-10-21 09:15:34]

alshabalin@yandex-team.ru changed reviewers:
+ danakj@chromium.org

[alshabalin, 2016-10-21 09:16:16]

PTAL

[danakj, 2016-10-21 21:05:02]

https://codereview.chromium.org/2434253003/diff/1/base/optional_unittest.cc
File base/optional_unittest.cc (right):

https://codereview.chromium.org/2434253003/diff/1/base/optional_unittest.cc#n...
base/optional_unittest.cc:99: static_assert(!base::Optional<bool>(),
"OptionalHasConstexprConstructor");
Can you add tests for the other constructors as well that should be constexpr?
http://en.cppreference.com/w/cpp/utility/optional/optional

[alshabalin, 2016-10-21 22:36:05]

https://codereview.chromium.org/2434253003/diff/1/base/optional_unittest.cc
File base/optional_unittest.cc (right):

https://codereview.chromium.org/2434253003/diff/1/base/optional_unittest.cc#n...
base/optional_unittest.cc:99: static_assert(!base::Optional<bool>(),
"OptionalHasConstexprConstructor");
On 2016/10/21 21:05:01, danakj wrote:
> Can you add tests for the other constructors as well that should be constexpr?
> http://en.cppreference.com/w/cpp/utility/optional/optional

Done (partially). Reading http://en.cppreference.com/w/cpp/language/constexpr it
seems that until C++14 constructors of union-like classes cannot be constexpr.
This means that only Optional of types with trivial special member functions can
have constexpr constructors.

[danakj, 2016-10-21 23:12:08]

LGTM w nits

https://codereview.chromium.org/2434253003/diff/40001/base/optional.h
File base/optional.h (right):

https://codereview.chromium.org/2434253003/diff/40001/base/optional.h#newcode45
base/optional.h:45: // When T is not trivially destructible we must call its
nit: whitespace above this to separate it since we have a lot of constructors
now

https://codereview.chromium.org/2434253003/diff/40001/base/optional.h#newcode72
base/optional.h:72: // When T is trivially destructible (i.e. its destructor
does nothing) there
same

https://codereview.chromium.org/2434253003/diff/40001/base/optional_unittest.cc
File base/optional_unittest.cc (right):

https://codereview.chromium.org/2434253003/diff/40001/base/optional_unittest....
base/optional_unittest.cc:201: constexpr float value = 0.1f;
Can you make all 3 cases in this test look alike? None of them need a first
Optional instance to call value() on really.

[alshabalin, 2016-10-22 07:47:52]

https://codereview.chromium.org/2434253003/diff/40001/base/optional.h
File base/optional.h (right):

https://codereview.chromium.org/2434253003/diff/40001/base/optional.h#newcode45
base/optional.h:45: // When T is not trivially destructible we must call its
On 2016/10/21 23:12:08, danakj wrote:
> nit: whitespace above this to separate it since we have a lot of constructors
> now

Done.

https://codereview.chromium.org/2434253003/diff/40001/base/optional.h#newcode72
base/optional.h:72: // When T is trivially destructible (i.e. its destructor
does nothing) there
On 2016/10/21 23:12:08, danakj wrote:
> same

Done.

https://codereview.chromium.org/2434253003/diff/40001/base/optional_unittest.cc
File base/optional_unittest.cc (right):

https://codereview.chromium.org/2434253003/diff/40001/base/optional_unittest....
base/optional_unittest.cc:201: constexpr float value = 0.1f;
On 2016/10/21 23:12:08, danakj wrote:
> Can you make all 3 cases in this test look alike? None of them need a first
> Optional instance to call value() on really.

Done.

[alshabalin, 2016-10-22 07:48:05]

The CQ bit was checked by alshabalin@yandex-team.ru

[alshabalin, 2016-10-22 07:48:05]

The patchset sent to the CQ was uploaded after l-g-t-m from danakj@chromium.org
Link to the patchset: https://codereview.chromium.org/2434253003/#ps60001
(title: "Address review comments.")

[commit-bot: I haz the power, 2016-10-22 07:48:16]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-22 08:12:50]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-22 08:12:51]

Try jobs failed on following builders:
  win_chromium_compile_dbg_ng on master.tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_comp...)

[alshabalin, 2016-10-22 22:10:30]

The CQ bit was checked by alshabalin@yandex-team.ru to run a CQ dry run

[commit-bot: I haz the power, 2016-10-22 22:10:41]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[alshabalin, 2016-10-22 22:16:26]

On 2016/10/21 21:05:02, danakj wrote:
> https://codereview.chromium.org/2434253003/diff/1/base/optional_unittest.cc
> File base/optional_unittest.cc (right):
> 
>
https://codereview.chromium.org/2434253003/diff/1/base/optional_unittest.cc#n...
> base/optional_unittest.cc:99: static_assert(!base::Optional<bool>(),
> "OptionalHasConstexprConstructor");
> Can you add tests for the other constructors as well that should be constexpr?
> http://en.cppreference.com/w/cpp/utility/optional/optional

More bad news: std::move and std::forward are constexpr only since C++14. So
constructors from T&& and the forwarding constructor can't be constexpr.

[alshabalin, 2016-10-22 22:47:38]

On 2016/10/22 22:16:26, alshabalin wrote:
> On 2016/10/21 21:05:02, danakj wrote:
> > https://codereview.chromium.org/2434253003/diff/1/base/optional_unittest.cc
> > File base/optional_unittest.cc (right):
> > 
> >
>
https://codereview.chromium.org/2434253003/diff/1/base/optional_unittest.cc#n...
> > base/optional_unittest.cc:99: static_assert(!base::Optional<bool>(),
> > "OptionalHasConstexprConstructor");
> > Can you add tests for the other constructors as well that should be
constexpr?
> > http://en.cppreference.com/w/cpp/utility/optional/optional
> 
> More bad news: std::move and std::forward are constexpr only since C++14. So
> constructors from T&& and the forwarding constructor can't be constexpr.

Also removed delegating constructor from Optional(base::nullopt_t): there seems
to be a bug
in MSVC 2015
http://stackoverflow.com/questions/32489702/constexpr-with-delegating-constru...

[alshabalin, 2016-10-22 22:48:01]

The CQ bit was checked by alshabalin@yandex-team.ru to run a CQ dry run

[commit-bot: I haz the power, 2016-10-22 22:48:15]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-23 01:42:17]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-23 01:42:18]

Dry run: Try jobs failed on following builders:
  android_arm64_dbg_recipe on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/android_arm6...)

[alshabalin, 2016-10-23 08:27:49]

Everything seems to work now. Could you please take another look?

[danakj, 2016-10-25 00:09:14]

LGTM thanks for the todos and investigations

[danakj, 2016-10-25 00:09:18]

The CQ bit was checked by danakj@chromium.org

[commit-bot: I haz the power, 2016-10-25 00:09:52]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-25 00:23:01]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-10-25 00:23:02]

Try jobs failed on following builders:
  ios-simulator on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator/bui...)

[alshabalin, 2016-10-25 06:04:50]

The CQ bit was checked by alshabalin@yandex-team.ru

[commit-bot: I haz the power, 2016-10-25 06:04:57]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-25 09:57:05]

Description was changed from

==========
Make base::Optional constructor constexpr.

BUG=
==========

to

==========
Make base::Optional constructor constexpr.

BUG=
==========

[commit-bot: I haz the power, 2016-10-25 09:57:06]

Committed patchset #6 (id:100001)

[commit-bot: I haz the power, 2016-10-25 09:58:25]

Description was changed from

==========
Make base::Optional constructor constexpr.

BUG=
==========

to

==========
Make base::Optional constructor constexpr.

BUG=

Committed: https://crrev.com/9494e454fa120c3d77f87c66142ef35f85ff225b
Cr-Commit-Position: refs/heads/master@{#427311}
==========

[commit-bot: I haz the power, 2016-10-25 09:58:26]

Patchset 6 (id:??) landed as
https://crrev.com/9494e454fa120c3d77f87c66142ef35f85ff225b
Cr-Commit-Position: refs/heads/master@{#427311}

[Mostyn Bramley-Moore, 2016-10-25 13:01:11]

This seems to have broken GCC (tested with 4.8) builds:

FAILED: obj/services/ui/public/interfaces/interfaces_cpp_sources/ime.mojom.o 
ccache g++ -MMD -MF
obj/services/ui/public/interfaces/interfaces_cpp_sources/ime.mojom.o.d
-DV8_DEPRECATION_WARNINGS -DENABLE_MDNS=1 -DENABLE_NOTIFICATIONS
-DENABLE_PLUGINS=1 -DENABLE_PDF=1 -DENABLE_SPELLCHECK=1
-DUI_COMPOSITOR_IMAGE_TRANSPORT -DUSE_AURA=1 -DUSE_DEFAULT_RENDER_THEME=1
-DUSE_NSS_CERTS=1 -DUSE_OZONE=1 -DDISABLE_NACL -DENABLE_EXTENSIONS=1
-DENABLE_TASK_MANAGER=1 -DENABLE_THEMES=1 -DENABLE_SESSION_SERVICE=1
-DENABLE_SUPERVISED_USERS=1 -DFULL_SAFE_BROWSING -DSAFE_BROWSING_CSD
-DSAFE_BROWSING_DB_LOCAL -DCHROMIUM_BUILD -DENABLE_MEDIA_ROUTER=1
-DFIELDTRIAL_TESTING_ENABLED -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE
-D_LARGEFILE64_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS
-D_FORTIFY_SOURCE=2 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0
-DGL_GLEXT_PROTOTYPES -DUSE_GLX -DUSE_EGL -DSK_IGNORE_DW_GRAY_FIX
-DSK_IGNORE_LINEONLY_AA_CONVEX_PATH_OPTS -DSK_SUPPORT_GPU=1
-DU_USING_ICU_NAMESPACE=0 -DU_ENABLE_DYLOAD=0 -DU_NOEXCEPT=
-DU_STATIC_IMPLEMENTATION -DICU_UTIL_DATA_IMPL=ICU_UTIL_DATA_FILE -I../.. -Igen
-I../../third_party/khronos -I../../gpu -I../../skia/config -I../../skia/ext
-I../../third_party/skia/include/c -I../../third_party/skia/include/config
-I../../third_party/skia/include/core -I../../third_party/skia/include/effects
-I../../third_party/skia/include/images -I../../third_party/skia/include/lazy
-I../../third_party/skia/include/pathops -I../../third_party/skia/include/pdf
-I../../third_party/skia/include/pipe -I../../third_party/skia/include/ports
-I../../third_party/skia/include/utils -I../../third_party/skia/include/gpu
-I../../third_party/skia/src/gpu -I../../third_party/skia/src/sksl
-I../../third_party/icu/source/common -I../../third_party/icu/source/i18n
-fno-strict-aliasing --param=ssp-buffer-size=4 -fstack-protector -funwind-tables
-fPIC -pipe -pthread -m64 -march=x86-64 -Wall -Werror -Wno-unused-local-typedefs
-Wno-maybe-uninitialized -Wno-missing-field-initializers -Wno-unused-parameter
-O2 -fno-ident -fdata-sections -ffunction-sections -g0 -fvisibility=hidden
-fno-threadsafe-statics -fvisibility-inlines-hidden -std=gnu++11 -Wno-narrowing
-fno-rtti -fno-exceptions -c gen/services/ui/public/interfaces/ime.mojom.cc -o
obj/services/ui/public/interfaces/interfaces_cpp_sources/ime.mojom.o
In file included from gen/services/ui/public/interfaces/ime.mojom.h:10:0,
                 from gen/services/ui/public/interfaces/ime.mojom.cc:15:
/usr/include/c++/4.8/type_traits: In instantiation of 'struct
std::is_convertible<std::default_delete<ui::Event>,
std::default_delete<ui::Event> >':
/usr/include/c++/4.8/type_traits:116:12:   required from 'struct
std::__and_<std::__not_<std::is_array<ui::Event> >,
std::is_convertible<std::default_delete<ui::Event>,
std::default_delete<ui::Event> > >'
/usr/include/c++/4.8/type_traits:121:12:   required from 'struct
std::__and_<std::is_convertible<ui::Event*, ui::Event*>,
std::__not_<std::is_array<ui::Event> >,
std::is_convertible<std::default_delete<ui::Event>,
std::default_delete<ui::Event> > >'
/usr/include/c++/4.8/type_traits:1775:71:   required by substitution of
'template<class ... _Cond> using _Require = typename std::enable_if<std::__and_<
<template-parameter-1-1> >::value>::type [with _Cond =
{std::is_convertible<ui::Event*, ui::Event*>,
std::__not_<std::is_array<ui::Event> >, std::conditional<false,
std::is_same<std::default_delete<ui::Event>, std::default_delete<ui::Event> >,
std::is_convertible<std::default_delete<ui::Event>,
std::default_delete<ui::Event> > >::type}]'
/usr/include/c++/4.8/bits/unique_ptr.h:163:44:   required from 'constexpr
base::internal::OptionalStorage<T, <anonymous> >::OptionalStorage() [with T =
std::unique_ptr<ui::Event>; bool <anonymous> = false]'
../../base/optional.h:119:13:   required from here
/usr/include/c++/4.8/type_traits:1317:12: error: the value of
'std::__is_convertible_helper<std::default_delete<ui::Event>,
std::default_delete<ui::Event>, false>::value' is not usable in a constant
expression
/usr/include/c++/4.8/type_traits:1312:29: note:
'std::__is_convertible_helper<std::default_delete<ui::Event>,
std::default_delete<ui::Event>, false>::value' used in its own initializer
/usr/include/c++/4.8/type_traits:1317:12: note: in template argument for type
'bool' 
gen/services/ui/public/interfaces/ime.mojom.cc: In constructor
'ui::mojom::CompositionEvent::CompositionEvent()':
gen/services/ui/public/interfaces/ime.mojom.cc:75:17: note: synthesized method
'constexpr base::Optional<T>::Optional() [with T = std::unique_ptr<ui::Event>]'
first required here

[danakj, 2016-10-25 20:18:09]

On 2016/10/25 13:01:11, Mostyn Bramley-Moore wrote:
> This seems to have broken GCC (tested with 4.8) builds:

Android bots use gcc, where are u getting this error?


> FAILED: obj/services/ui/public/interfaces/interfaces_cpp_sources/ime.mojom.o 
> ccache g++ -MMD -MF
> obj/services/ui/public/interfaces/interfaces_cpp_sources/ime.mojom.o.d
> -DV8_DEPRECATION_WARNINGS -DENABLE_MDNS=1 -DENABLE_NOTIFICATIONS
> -DENABLE_PLUGINS=1 -DENABLE_PDF=1 -DENABLE_SPELLCHECK=1
> -DUI_COMPOSITOR_IMAGE_TRANSPORT -DUSE_AURA=1 -DUSE_DEFAULT_RENDER_THEME=1
> -DUSE_NSS_CERTS=1 -DUSE_OZONE=1 -DDISABLE_NACL -DENABLE_EXTENSIONS=1
> -DENABLE_TASK_MANAGER=1 -DENABLE_THEMES=1 -DENABLE_SESSION_SERVICE=1
> -DENABLE_SUPERVISED_USERS=1 -DFULL_SAFE_BROWSING -DSAFE_BROWSING_CSD
> -DSAFE_BROWSING_DB_LOCAL -DCHROMIUM_BUILD -DENABLE_MEDIA_ROUTER=1
> -DFIELDTRIAL_TESTING_ENABLED -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE
> -D_LARGEFILE64_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS
> -D_FORTIFY_SOURCE=2 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0
> -DGL_GLEXT_PROTOTYPES -DUSE_GLX -DUSE_EGL -DSK_IGNORE_DW_GRAY_FIX
> -DSK_IGNORE_LINEONLY_AA_CONVEX_PATH_OPTS -DSK_SUPPORT_GPU=1
> -DU_USING_ICU_NAMESPACE=0 -DU_ENABLE_DYLOAD=0 -DU_NOEXCEPT=
> -DU_STATIC_IMPLEMENTATION -DICU_UTIL_DATA_IMPL=ICU_UTIL_DATA_FILE -I../..
-Igen
> -I../../third_party/khronos -I../../gpu -I../../skia/config -I../../skia/ext
> -I../../third_party/skia/include/c -I../../third_party/skia/include/config
> -I../../third_party/skia/include/core -I../../third_party/skia/include/effects
> -I../../third_party/skia/include/images -I../../third_party/skia/include/lazy
> -I../../third_party/skia/include/pathops -I../../third_party/skia/include/pdf
> -I../../third_party/skia/include/pipe -I../../third_party/skia/include/ports
> -I../../third_party/skia/include/utils -I../../third_party/skia/include/gpu
> -I../../third_party/skia/src/gpu -I../../third_party/skia/src/sksl
> -I../../third_party/icu/source/common -I../../third_party/icu/source/i18n
> -fno-strict-aliasing --param=ssp-buffer-size=4 -fstack-protector
-funwind-tables
> -fPIC -pipe -pthread -m64 -march=x86-64 -Wall -Werror
-Wno-unused-local-typedefs
> -Wno-maybe-uninitialized -Wno-missing-field-initializers -Wno-unused-parameter
> -O2 -fno-ident -fdata-sections -ffunction-sections -g0 -fvisibility=hidden
> -fno-threadsafe-statics -fvisibility-inlines-hidden -std=gnu++11
-Wno-narrowing
> -fno-rtti -fno-exceptions -c gen/services/ui/public/interfaces/ime.mojom.cc -o
> obj/services/ui/public/interfaces/interfaces_cpp_sources/ime.mojom.o
> In file included from gen/services/ui/public/interfaces/ime.mojom.h:10:0,
>                  from gen/services/ui/public/interfaces/ime.mojom.cc:15:
> /usr/include/c++/4.8/type_traits: In instantiation of 'struct
> std::is_convertible<std::default_delete<ui::Event>,
> std::default_delete<ui::Event> >':
> /usr/include/c++/4.8/type_traits:116:12:   required from 'struct
> std::__and_<std::__not_<std::is_array<ui::Event> >,
> std::is_convertible<std::default_delete<ui::Event>,
> std::default_delete<ui::Event> > >'
> /usr/include/c++/4.8/type_traits:121:12:   required from 'struct
> std::__and_<std::is_convertible<ui::Event*, ui::Event*>,
> std::__not_<std::is_array<ui::Event> >,
> std::is_convertible<std::default_delete<ui::Event>,
> std::default_delete<ui::Event> > >'
> /usr/include/c++/4.8/type_traits:1775:71:   required by substitution of
> 'template<class ... _Cond> using _Require = typename
std::enable_if<std::__and_<
> <template-parameter-1-1> >::value>::type [with _Cond =
> {std::is_convertible<ui::Event*, ui::Event*>,
> std::__not_<std::is_array<ui::Event> >, std::conditional<false,
> std::is_same<std::default_delete<ui::Event>, std::default_delete<ui::Event> >,
> std::is_convertible<std::default_delete<ui::Event>,
> std::default_delete<ui::Event> > >::type}]'
> /usr/include/c++/4.8/bits/unique_ptr.h:163:44:   required from 'constexpr
> base::internal::OptionalStorage<T, <anonymous> >::OptionalStorage() [with T =
> std::unique_ptr<ui::Event>; bool <anonymous> = false]'
> ../../base/optional.h:119:13:   required from here
> /usr/include/c++/4.8/type_traits:1317:12: error: the value of
> 'std::__is_convertible_helper<std::default_delete<ui::Event>,
> std::default_delete<ui::Event>, false>::value' is not usable in a constant
> expression
> /usr/include/c++/4.8/type_traits:1312:29: note:
> 'std::__is_convertible_helper<std::default_delete<ui::Event>,
> std::default_delete<ui::Event>, false>::value' used in its own initializer
> /usr/include/c++/4.8/type_traits:1317:12: note: in template argument for type
> 'bool' 
> gen/services/ui/public/interfaces/ime.mojom.cc: In constructor
> 'ui::mojom::CompositionEvent::CompositionEvent()':
> gen/services/ui/public/interfaces/ime.mojom.cc:75:17: note: synthesized method
> 'constexpr base::Optional<T>::Optional() [with T =
std::unique_ptr<ui::Event>]'
> first required here

[Mostyn Bramley-Moore, 2016-10-25 22:13:25]

On 2016/10/25 20:18:09, danakj wrote:
> On 2016/10/25 13:01:11, Mostyn Bramley-Moore wrote:
> > This seems to have broken GCC (tested with 4.8) builds:
> 
> Android bots use gcc, where are u getting this error?

I am building on ubuntu 14.04.  This seems to be sufficient to reproduce the
error:

rm -rf out_foo
gn gen out_foo --args="cc_wrapper=\"ccache\" is_clang=false use_sysroot=false" #
the cc_wrapper part is optional
ninja -C out_foo
obj/services/ui/public/interfaces/interfaces_cpp_sources/ime.mojom.o

[alshabalin, 2016-10-26 11:32:38]

On 2016/10/25 22:13:25, Mostyn Bramley-Moore wrote:
> On 2016/10/25 20:18:09, danakj wrote:
> > On 2016/10/25 13:01:11, Mostyn Bramley-Moore wrote:
> > > This seems to have broken GCC (tested with 4.8) builds:
> > 
> > Android bots use gcc, where are u getting this error?
> 
> I am building on ubuntu 14.04.  This seems to be sufficient to reproduce the
> error:
> 
> rm -rf out_foo
> gn gen out_foo --args="cc_wrapper=\"ccache\" is_clang=false use_sysroot=false"
#
> the cc_wrapper part is optional
> ninja -C out_foo
> obj/services/ui/public/interfaces/interfaces_cpp_sources/ime.mojom.o

I managed to reproduce with this code snippet:

#include <memory>

class C {
 public:
  constexpr C() {}
  ~C() {}

 private:
  union {
    char empty_ = '\0';
    std::unique_ptr<int> value_;
  };
};

int main() {
  return 0;
}

g++ 4.8 fails to compile while g++ 6.2 compiles successfully. I didn't find
any explanation as to why but adding empty_('\0') to the member initializer list
made g++ 4.8 happy.

[alshabalin, 2016-10-26 11:47:50]

On 2016/10/25 13:01:11, Mostyn Bramley-Moore wrote:
> This seems to have broken GCC (tested with 4.8) builds:
> 
> FAILED: obj/services/ui/public/interfaces/interfaces_cpp_sources/ime.mojom.o 
> ccache g++ -MMD -MF
> obj/services/ui/public/interfaces/interfaces_cpp_sources/ime.mojom.o.d
> -DV8_DEPRECATION_WARNINGS -DENABLE_MDNS=1 -DENABLE_NOTIFICATIONS
> -DENABLE_PLUGINS=1 -DENABLE_PDF=1 -DENABLE_SPELLCHECK=1
> -DUI_COMPOSITOR_IMAGE_TRANSPORT -DUSE_AURA=1 -DUSE_DEFAULT_RENDER_THEME=1
> -DUSE_NSS_CERTS=1 -DUSE_OZONE=1 -DDISABLE_NACL -DENABLE_EXTENSIONS=1
> -DENABLE_TASK_MANAGER=1 -DENABLE_THEMES=1 -DENABLE_SESSION_SERVICE=1
> -DENABLE_SUPERVISED_USERS=1 -DFULL_SAFE_BROWSING -DSAFE_BROWSING_CSD
> -DSAFE_BROWSING_DB_LOCAL -DCHROMIUM_BUILD -DENABLE_MEDIA_ROUTER=1
> -DFIELDTRIAL_TESTING_ENABLED -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE
> -D_LARGEFILE64_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS
> -D_FORTIFY_SOURCE=2 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0
> -DGL_GLEXT_PROTOTYPES -DUSE_GLX -DUSE_EGL -DSK_IGNORE_DW_GRAY_FIX
> -DSK_IGNORE_LINEONLY_AA_CONVEX_PATH_OPTS -DSK_SUPPORT_GPU=1
> -DU_USING_ICU_NAMESPACE=0 -DU_ENABLE_DYLOAD=0 -DU_NOEXCEPT=
> -DU_STATIC_IMPLEMENTATION -DICU_UTIL_DATA_IMPL=ICU_UTIL_DATA_FILE -I../..
-Igen
> -I../../third_party/khronos -I../../gpu -I../../skia/config -I../../skia/ext
> -I../../third_party/skia/include/c -I../../third_party/skia/include/config
> -I../../third_party/skia/include/core -I../../third_party/skia/include/effects
> -I../../third_party/skia/include/images -I../../third_party/skia/include/lazy
> -I../../third_party/skia/include/pathops -I../../third_party/skia/include/pdf
> -I../../third_party/skia/include/pipe -I../../third_party/skia/include/ports
> -I../../third_party/skia/include/utils -I../../third_party/skia/include/gpu
> -I../../third_party/skia/src/gpu -I../../third_party/skia/src/sksl
> -I../../third_party/icu/source/common -I../../third_party/icu/source/i18n
> -fno-strict-aliasing --param=ssp-buffer-size=4 -fstack-protector
-funwind-tables
> -fPIC -pipe -pthread -m64 -march=x86-64 -Wall -Werror
-Wno-unused-local-typedefs
> -Wno-maybe-uninitialized -Wno-missing-field-initializers -Wno-unused-parameter
> -O2 -fno-ident -fdata-sections -ffunction-sections -g0 -fvisibility=hidden
> -fno-threadsafe-statics -fvisibility-inlines-hidden -std=gnu++11
-Wno-narrowing
> -fno-rtti -fno-exceptions -c gen/services/ui/public/interfaces/ime.mojom.cc -o
> obj/services/ui/public/interfaces/interfaces_cpp_sources/ime.mojom.o
> In file included from gen/services/ui/public/interfaces/ime.mojom.h:10:0,
>                  from gen/services/ui/public/interfaces/ime.mojom.cc:15:
> /usr/include/c++/4.8/type_traits: In instantiation of 'struct
> std::is_convertible<std::default_delete<ui::Event>,
> std::default_delete<ui::Event> >':
> /usr/include/c++/4.8/type_traits:116:12:   required from 'struct
> std::__and_<std::__not_<std::is_array<ui::Event> >,
> std::is_convertible<std::default_delete<ui::Event>,
> std::default_delete<ui::Event> > >'
> /usr/include/c++/4.8/type_traits:121:12:   required from 'struct
> std::__and_<std::is_convertible<ui::Event*, ui::Event*>,
> std::__not_<std::is_array<ui::Event> >,
> std::is_convertible<std::default_delete<ui::Event>,
> std::default_delete<ui::Event> > >'
> /usr/include/c++/4.8/type_traits:1775:71:   required by substitution of
> 'template<class ... _Cond> using _Require = typename
std::enable_if<std::__and_<
> <template-parameter-1-1> >::value>::type [with _Cond =
> {std::is_convertible<ui::Event*, ui::Event*>,
> std::__not_<std::is_array<ui::Event> >, std::conditional<false,
> std::is_same<std::default_delete<ui::Event>, std::default_delete<ui::Event> >,
> std::is_convertible<std::default_delete<ui::Event>,
> std::default_delete<ui::Event> > >::type}]'
> /usr/include/c++/4.8/bits/unique_ptr.h:163:44:   required from 'constexpr
> base::internal::OptionalStorage<T, <anonymous> >::OptionalStorage() [with T =
> std::unique_ptr<ui::Event>; bool <anonymous> = false]'
> ../../base/optional.h:119:13:   required from here
> /usr/include/c++/4.8/type_traits:1317:12: error: the value of
> 'std::__is_convertible_helper<std::default_delete<ui::Event>,
> std::default_delete<ui::Event>, false>::value' is not usable in a constant
> expression
> /usr/include/c++/4.8/type_traits:1312:29: note:
> 'std::__is_convertible_helper<std::default_delete<ui::Event>,
> std::default_delete<ui::Event>, false>::value' used in its own initializer
> /usr/include/c++/4.8/type_traits:1317:12: note: in template argument for type
> 'bool' 
> gen/services/ui/public/interfaces/ime.mojom.cc: In constructor
> 'ui::mojom::CompositionEvent::CompositionEvent()':
> gen/services/ui/public/interfaces/ime.mojom.cc:75:17: note: synthesized method
> 'constexpr base::Optional<T>::Optional() [with T =
std::unique_ptr<ui::Event>]'
> first required here

I've made a follow up CL fixing this:
https://codereview.chromium.org/2453733002/