PR libstdc++/81468 constrain std::chrono::time_point constructor

Message ID 20170913140903.GA8606@redhat.com
State New
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  • PR libstdc++/81468 constrain std::chrono::time_point constructor
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Commit Message

Jonathan Wakely Sept. 13, 2017, 2:09 p.m.
Howard reported this bug, caused by a missing SFINAE constraint on a
std::chrono::time_point constructor.

I've also done a bit of simplification using alias templates.

	PR libstdc++/81468
	* include/std/chrono (__enable_if_is_duration)
	(__disable_if_is_duration): New alias templates to simplify SFINAE.
	(duration_cast, floor, ceil): Use __enable_if_is_duration.
	(duration::__is_float, duration::__is_harmonic): New alias templates
	to simplify SFINAE.
	(duration::duration(const _Rep2&)): Use _Require, __is_float and
	__is_harmonic.
	(duration::duration(const duration<_Rep2, _Period2>&)): Likewise.
	(__common_rep_type): Remove, replace with ...
	(__common_rep_t): New alias template.
	(operator*, operator/, operator%): Use __common_rep_t and
	__disable_if_is_duration.
	(time_point::time_point(const time_point<clock, _Dur2>&)): Add missing
	constraint from LWG DR 1177.
	* testsuite/20_util/duration/cons/dr1177.cc: New.
	* testsuite/20_util/duration/literals/range.cc: Update dg-error line.
	* testsuite/20_util/duration/requirements/typedefs_neg1.cc: Likewise.
	* testsuite/20_util/duration/requirements/typedefs_neg2.cc: Likewise.
	* testsuite/20_util/duration/requirements/typedefs_neg3.cc: Likewise.
	* testsuite/20_util/time_point/cons/81468.cc: New.

Tested on powerp64le-linux, committed to trunk.
commit e06db0fa47676eeb386bfd1a582a005d341fb678
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 13 13:00:50 2017 +0100

    PR libstdc++/81468 constrain std::chrono::time_point constructor
    
            PR libstdc++/81468
            * include/std/chrono (__enable_if_is_duration)
            (__disable_if_is_duration): New alias templates to simplify SFINAE.
            (duration_cast, floor, ceil): Use __enable_if_is_duration.
            (duration::__is_float, duration::__is_harmonic): New alias templates
            to simplify SFINAE.
            (duration::duration(const _Rep2&)): Use _Require, __is_float and
            __is_harmonic.
            (duration::duration(const duration<_Rep2, _Period2>&)): Likewise.
            (__common_rep_type): Remove, replace with ...
            (__common_rep_t): New alias template.
            (operator*, operator/, operator%): Use __common_rep_t and
            __disable_if_is_duration.
            (time_point::time_point(const time_point<clock, _Dur2>&)): Add missing
            constraint from LWG DR 1177.
            * testsuite/20_util/duration/cons/dr1177.cc: New.
            * testsuite/20_util/duration/literals/range.cc: Update dg-error line.
            * testsuite/20_util/duration/requirements/typedefs_neg1.cc: Likewise.
            * testsuite/20_util/duration/requirements/typedefs_neg2.cc: Likewise.
            * testsuite/20_util/duration/requirements/typedefs_neg3.cc: Likewise.
            * testsuite/20_util/time_point/cons/81468.cc: New.

Comments

Jonathan Wakely Sept. 13, 2017, 2:55 p.m. | #1
On 13/09/17 15:09 +0100, Jonathan Wakely wrote:
>Howard reported this bug, caused by a missing SFINAE constraint on a
>std::chrono::time_point constructor.
>
>I've also done a bit of simplification using alias templates.

Here's the backport for the branches, which just adds the constraint
(and tests) without the new alias templates.
commit 4a6c846f3203dd0ceb13b7130e686833a4150d96
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 13 15:16:31 2017 +0100

    PR libstdc++/81468 constrain std::chrono::time_point constructor
    
            PR libstdc++/81468
            * include/std/chrono (time_point(const time_point<_Dur2>&)): Add
            missing constraint from LWG DR 1177.
            * testsuite/20_util/duration/cons/dr1177.cc: New.
            * testsuite/20_util/time_point/cons/81468.cc: New.
            * testsuite/20_util/duration/literals/range.cc: Update dg-error line.

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index c3a6ba8f873..cc63d44657b 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -622,7 +622,8 @@ _GLIBCXX_END_NAMESPACE_VERSION
 	{ }
 
 	// conversions
-	template<typename _Dur2>
+	template<typename _Dur2,
+		 typename = _Require<is_convertible<_Dur2, _Dur>>>
 	  constexpr time_point(const time_point<clock, _Dur2>& __t)
 	  : __d(__t.time_since_epoch())
 	  { }
diff --git a/libstdc++-v3/testsuite/20_util/duration/cons/dr1177.cc b/libstdc++-v3/testsuite/20_util/duration/cons/dr1177.cc
new file mode 100644
index 00000000000..28c881ccc79
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/duration/cons/dr1177.cc
@@ -0,0 +1,41 @@
+// Copyright (C) 2017 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do compile { target c++11 } }
+
+#include <chrono>
+#include <type_traits>
+
+using namespace std;
+using namespace std::chrono;
+
+// DR 1177
+static_assert(is_constructible<duration<float>, duration<double>>{},
+    "can convert duration with one floating point rep to another");
+static_assert(is_constructible<duration<float>, duration<int>>{},
+    "can convert duration with integral rep to one with floating point rep");
+static_assert(!is_constructible<duration<int>, duration<float>>{},
+    "cannot convert duration with floating point rep to one with integral rep");
+static_assert(is_constructible<duration<int>, duration<long>>{},
+    "can convert duration with one integral rep to another");
+
+static_assert(!is_constructible<duration<int>, duration<int, ratio<2,3>>>{},
+    "cannot convert duration to one with different period");
+static_assert(is_constructible<duration<float>, duration<int, ratio<2,3>>>{},
+    "unless it has a floating-point representation");
+static_assert(is_constructible<duration<float>, duration<int, ratio<1,3>>>{},
+    "or a period that is an integral multiple of the original");
diff --git a/libstdc++-v3/testsuite/20_util/duration/literals/range.cc b/libstdc++-v3/testsuite/20_util/duration/literals/range.cc
index c0d1a6e5885..531b53c42ec 100644
--- a/libstdc++-v3/testsuite/20_util/duration/literals/range.cc
+++ b/libstdc++-v3/testsuite/20_util/duration/literals/range.cc
@@ -26,6 +26,6 @@ test01()
 
   // std::numeric_limits<int64_t>::max() == 9223372036854775807;
   auto h = 9223372036854775808h;
-  // { dg-error "cannot be represented" "" { target *-*-* } 892 }
+  // { dg-error "cannot be represented" "" { target *-*-* } 893 }
 }
 // { dg-prune-output "in constexpr expansion" } // needed for -O0
diff --git a/libstdc++-v3/testsuite/20_util/time_point/cons/81468.cc b/libstdc++-v3/testsuite/20_util/time_point/cons/81468.cc
new file mode 100644
index 00000000000..30d1c4a5ac7
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/time_point/cons/81468.cc
@@ -0,0 +1,34 @@
+// Copyright (C) 2017 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do compile { target c++11 } }
+
+#include <chrono>
+#include <type_traits>
+
+using namespace std;
+using namespace std::chrono;
+
+template <class Duration>
+    using sys_time = time_point<system_clock, Duration>;
+
+static_assert(is_constructible<sys_time<milliseconds>, sys_time<seconds>>{},
+    "Can construct time_point from one with lower precision duration");
+
+// PR libstdc++/81468 - DR 1177
+static_assert(!is_constructible<sys_time<seconds>, sys_time<milliseconds>>{},
+    "Cannot construct time_point from one with higher precision duration");
Tim Song Sept. 14, 2017, 1:30 a.m. | #2
On Wed, Sep 13, 2017 at 10:55 AM, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> +// DR 1177
> +static_assert(is_constructible<duration<float>, duration<double>>{},
> +    "can convert duration with one floating point rep to another");
> +static_assert(is_constructible<duration<float>, duration<int>>{},
> +    "can convert duration with integral rep to one with floating point rep");
> +static_assert(!is_constructible<duration<int>, duration<float>>{},
> +    "cannot convert duration with floating point rep to one with integral rep");
> +static_assert(is_constructible<duration<int>, duration<long>>{},
> +    "can convert duration with one integral rep to another");
> +
> +static_assert(!is_constructible<duration<int>, duration<int, ratio<2,3>>>{},
> +    "cannot convert duration to one with different period");
> +static_assert(is_constructible<duration<float>, duration<int, ratio<2,3>>>{},
> +    "unless it has a floating-point representation");

"it" is a little ambiguous here unless you read the next message's
mention of "the original"...

> +static_assert(is_constructible<duration<float>, duration<int, ratio<1,3>>>{},
> +    "or a period that is an integral multiple of the original");

This is backwards: duration<Inty, P1> is convertible to duration<Inty,
P2> iff P1 is an integral multiple of P2, i.e., if the original's
period is an integral multiple of "its" period.

The static assert only passed because duration<float> was used as the
destination type (presumably because of a copy/paste error).

Tim
Jonathan Wakely Sept. 18, 2017, 8 p.m. | #3
On 13/09/17 21:30 -0400, Tim Song wrote:
>On Wed, Sep 13, 2017 at 10:55 AM, Jonathan Wakely <jwakely@redhat.com> wrote:
>>
>> +// DR 1177
>> +static_assert(is_constructible<duration<float>, duration<double>>{},
>> +    "can convert duration with one floating point rep to another");
>> +static_assert(is_constructible<duration<float>, duration<int>>{},
>> +    "can convert duration with integral rep to one with floating point rep");
>> +static_assert(!is_constructible<duration<int>, duration<float>>{},
>> +    "cannot convert duration with floating point rep to one with integral rep");
>> +static_assert(is_constructible<duration<int>, duration<long>>{},
>> +    "can convert duration with one integral rep to another");
>> +
>> +static_assert(!is_constructible<duration<int>, duration<int, ratio<2,3>>>{},
>> +    "cannot convert duration to one with different period");
>> +static_assert(is_constructible<duration<float>, duration<int, ratio<2,3>>>{},
>> +    "unless it has a floating-point representation");
>
>"it" is a little ambiguous here unless you read the next message's
>mention of "the original"...
>
>> +static_assert(is_constructible<duration<float>, duration<int, ratio<1,3>>>{},
>> +    "or a period that is an integral multiple of the original");
>
>This is backwards: duration<Inty, P1> is convertible to duration<Inty,
>P2> iff P1 is an integral multiple of P2, i.e., if the original's
>period is an integral multiple of "its" period.
>
>The static assert only passed because duration<float> was used as the
>destination type (presumably because of a copy/paste error).
>
>Tim

Good catch, thanks.

I've committed this patch.
commit 5c021e19e0758e5ad7e47feadbd0632b15f85785
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Sep 18 19:04:25 2017 +0100

    PR libstdc++/81468 fix test for duration conversions
    
            PR libstdc++/81468
            * testsuite/20_util/duration/cons/dr1177.cc: Fix incorrect test and
            improve static assertion messages.

diff --git a/libstdc++-v3/testsuite/20_util/duration/cons/dr1177.cc b/libstdc++-v3/testsuite/20_util/duration/cons/dr1177.cc
index 28c881ccc79..d90cd27f482 100644
--- a/libstdc++-v3/testsuite/20_util/duration/cons/dr1177.cc
+++ b/libstdc++-v3/testsuite/20_util/duration/cons/dr1177.cc
@@ -36,6 +36,6 @@ static_assert(is_constructible<duration<int>, duration<long>>{},
 static_assert(!is_constructible<duration<int>, duration<int, ratio<2,3>>>{},
     "cannot convert duration to one with different period");
 static_assert(is_constructible<duration<float>, duration<int, ratio<2,3>>>{},
-    "unless it has a floating-point representation");
-static_assert(is_constructible<duration<float>, duration<int, ratio<1,3>>>{},
-    "or a period that is an integral multiple of the original");
+    "... unless the result type has a floating-point representation");
+static_assert(is_constructible<duration<int, ratio<1,3>>, duration<int>>{},
+    "... or the original's period is a multiple of the result's period");

Patch

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 1bcbf524a7b..fc058fcd8d8 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -179,10 +179,17 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       : std::true_type
       { };
 
+    template<typename _Tp>
+      using __enable_if_is_duration
+	= typename enable_if<__is_duration<_Tp>::value, _Tp>::type;
+
+    template<typename _Tp>
+      using __disable_if_is_duration
+	= typename enable_if<!__is_duration<_Tp>::value, _Tp>::type;
+
     /// duration_cast
     template<typename _ToDur, typename _Rep, typename _Period>
-      constexpr typename enable_if<__is_duration<_ToDur>::value,
-				   _ToDur>::type
+      constexpr __enable_if_is_duration<_ToDur>
       duration_cast(const duration<_Rep, _Period>& __d)
       {
 	typedef typename _ToDur::period				__to_period;
@@ -211,7 +218,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 # define __cpp_lib_chrono 201510
 
     template<typename _ToDur, typename _Rep, typename _Period>
-      constexpr enable_if_t<__is_duration<_ToDur>::value, _ToDur>
+      constexpr __enable_if_is_duration<_ToDur>
       floor(const duration<_Rep, _Period>& __d)
       {
 	auto __to = chrono::duration_cast<_ToDur>(__d);
@@ -221,7 +228,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
     template<typename _ToDur, typename _Rep, typename _Period>
-      constexpr enable_if_t<__is_duration<_ToDur>::value, _ToDur>
+      constexpr __enable_if_is_duration<_ToDur>
       ceil(const duration<_Rep, _Period>& __d)
       {
 	auto __to = chrono::duration_cast<_ToDur>(__d);
@@ -294,6 +301,17 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     template<typename _Rep, typename _Period>
       struct duration
       {
+      private:
+	template<typename _Rep2>
+	  using __is_float = treat_as_floating_point<_Rep2>;
+
+	// _Period2 is an exact multiple of _Period
+	template<typename _Period2>
+	  using __is_harmonic
+	    = __bool_constant<ratio_divide<_Period2, _Period>::den == 1>;
+
+      public:
+
 	typedef _Rep						rep;
 	typedef _Period 					period;
 
@@ -305,22 +323,18 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	// 20.11.5.1 construction / copy / destroy
 	constexpr duration() = default;
 
-	// NB: Make constexpr implicit. This cannot be explicitly
-	// constexpr, as any UDT that is not a literal type with a
-	// constexpr copy constructor will be ill-formed.
 	duration(const duration&) = default;
 
-	template<typename _Rep2, typename = typename
-	       enable_if<is_convertible<_Rep2, rep>::value
-			 && (treat_as_floating_point<rep>::value
-			     || !treat_as_floating_point<_Rep2>::value)>::type>
+	template<typename _Rep2, typename = _Require<
+		 is_convertible<_Rep2, rep>,
+		 __or_<__is_float<rep>, __not_<__is_float<_Rep2>>>>>
 	  constexpr explicit duration(const _Rep2& __rep)
 	  : __r(static_cast<rep>(__rep)) { }
 
-	template<typename _Rep2, typename _Period2, typename = typename
-	       enable_if<treat_as_floating_point<rep>::value
-			 || (ratio_divide<_Period2, period>::den == 1
-			     && !treat_as_floating_point<_Rep2>::value)>::type>
+	template<typename _Rep2, typename _Period2, typename = _Require<
+		 __or_<__is_float<rep>,
+		       __and_<__is_harmonic<_Period2>,
+			      __not_<__is_float<_Rep2>>>>>>
 	  constexpr duration(const duration<_Rep2, _Period2>& __d)
 	  : __r(duration_cast<duration>(__d).count()) { }
 
@@ -455,18 +469,15 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return __cd(__cd(__lhs).count() - __cd(__rhs).count());
       }
 
-    template<typename _Rep1, typename _Rep2, bool =
-	     is_convertible<_Rep2,
-			    typename common_type<_Rep1, _Rep2>::type>::value>
-      struct __common_rep_type { };
-
-    template<typename _Rep1, typename _Rep2>
-      struct __common_rep_type<_Rep1, _Rep2, true>
-      { typedef typename common_type<_Rep1, _Rep2>::type type; };
+    // SFINAE helper to obtain common_type<_Rep1, _Rep2> only if _Rep2
+    // is implicitly convertible to it.
+    template<typename _Rep1, typename _Rep2,
+	     typename _CRep = typename common_type<_Rep1, _Rep2>::type>
+      using __common_rep_t
+	= typename enable_if<is_convertible<_Rep2, _CRep>::value, _CRep>::type;
 
     template<typename _Rep1, typename _Period, typename _Rep2>
-      constexpr
-      duration<typename __common_rep_type<_Rep1, _Rep2>::type, _Period>
+      constexpr duration<__common_rep_t<_Rep1, _Rep2>, _Period>
       operator*(const duration<_Rep1, _Period>& __d, const _Rep2& __s)
       {
 	typedef duration<typename common_type<_Rep1, _Rep2>::type, _Period>
@@ -475,14 +486,13 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
     template<typename _Rep1, typename _Rep2, typename _Period>
-      constexpr
-      duration<typename __common_rep_type<_Rep2, _Rep1>::type, _Period>
+      constexpr duration<__common_rep_t<_Rep2, _Rep1>, _Period>
       operator*(const _Rep1& __s, const duration<_Rep2, _Period>& __d)
       { return __d * __s; }
 
     template<typename _Rep1, typename _Period, typename _Rep2>
-      constexpr duration<typename __common_rep_type<_Rep1, typename
-	enable_if<!__is_duration<_Rep2>::value, _Rep2>::type>::type, _Period>
+      constexpr
+      duration<__common_rep_t<_Rep1, __disable_if_is_duration<_Rep2>>, _Period>
       operator/(const duration<_Rep1, _Period>& __d, const _Rep2& __s)
       {
 	typedef duration<typename common_type<_Rep1, _Rep2>::type, _Period>
@@ -504,8 +514,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
     // DR 934.
     template<typename _Rep1, typename _Period, typename _Rep2>
-      constexpr duration<typename __common_rep_type<_Rep1, typename
-	enable_if<!__is_duration<_Rep2>::value, _Rep2>::type>::type, _Period>
+      constexpr
+      duration<__common_rep_t<_Rep1, __disable_if_is_duration<_Rep2>>, _Period>
       operator%(const duration<_Rep1, _Period>& __d, const _Rep2& __s)
       {
 	typedef duration<typename common_type<_Rep1, _Rep2>::type, _Period>
@@ -614,7 +624,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	{ }
 
 	// conversions
-	template<typename _Dur2>
+	template<typename _Dur2,
+		 typename = _Require<is_convertible<_Dur2, _Dur>>>
 	  constexpr time_point(const time_point<clock, _Dur2>& __t)
 	  : __d(__t.time_since_epoch())
 	  { }
diff --git a/libstdc++-v3/testsuite/20_util/duration/cons/dr1177.cc b/libstdc++-v3/testsuite/20_util/duration/cons/dr1177.cc
new file mode 100644
index 00000000000..28c881ccc79
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/duration/cons/dr1177.cc
@@ -0,0 +1,41 @@ 
+// Copyright (C) 2017 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do compile { target c++11 } }
+
+#include <chrono>
+#include <type_traits>
+
+using namespace std;
+using namespace std::chrono;
+
+// DR 1177
+static_assert(is_constructible<duration<float>, duration<double>>{},
+    "can convert duration with one floating point rep to another");
+static_assert(is_constructible<duration<float>, duration<int>>{},
+    "can convert duration with integral rep to one with floating point rep");
+static_assert(!is_constructible<duration<int>, duration<float>>{},
+    "cannot convert duration with floating point rep to one with integral rep");
+static_assert(is_constructible<duration<int>, duration<long>>{},
+    "can convert duration with one integral rep to another");
+
+static_assert(!is_constructible<duration<int>, duration<int, ratio<2,3>>>{},
+    "cannot convert duration to one with different period");
+static_assert(is_constructible<duration<float>, duration<int, ratio<2,3>>>{},
+    "unless it has a floating-point representation");
+static_assert(is_constructible<duration<float>, duration<int, ratio<1,3>>>{},
+    "or a period that is an integral multiple of the original");
diff --git a/libstdc++-v3/testsuite/20_util/duration/literals/range.cc b/libstdc++-v3/testsuite/20_util/duration/literals/range.cc
index b5105dff38b..36e71eea72b 100644
--- a/libstdc++-v3/testsuite/20_util/duration/literals/range.cc
+++ b/libstdc++-v3/testsuite/20_util/duration/literals/range.cc
@@ -26,6 +26,6 @@  test01()
 
   // std::numeric_limits<int64_t>::max() == 9223372036854775807;
   auto h = 9223372036854775808h;
-  // { dg-error "cannot be represented" "" { target *-*-* } 880 }
+  // { dg-error "cannot be represented" "" { target *-*-* } 891 }
 }
 // { dg-prune-output "in constexpr expansion" } // needed for -O0
diff --git a/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg1.cc b/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg1.cc
index 17f74187098..d57f6dedd66 100644
--- a/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg1.cc
+++ b/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg1.cc
@@ -30,4 +30,4 @@  void test01()
   test_type d; // { dg-error "required from here" }
 }
 
-// { dg-error "rep cannot be a duration" "" { target *-*-* } 300 }
+// { dg-error "rep cannot be a duration" "" { target *-*-* } 318 }
diff --git a/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg2.cc b/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg2.cc
index 282a9dbdc85..39283b2cb74 100644
--- a/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg2.cc
+++ b/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg2.cc
@@ -31,5 +31,5 @@  void test01()
   test_type d;			// { dg-error "required from here" }
 }
 
-// { dg-error "must be a specialization of ratio" "" { target *-*-* } 301 }
+// { dg-error "must be a specialization of ratio" "" { target *-*-* } 319 }
 // { dg-prune-output "not a member" }
diff --git a/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg3.cc b/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg3.cc
index c57f9d947a5..9beaa83fbfa 100644
--- a/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg3.cc
+++ b/libstdc++-v3/testsuite/20_util/duration/requirements/typedefs_neg3.cc
@@ -32,4 +32,4 @@  void test01()
   test_type d;  // { dg-error "required from here" }
 }
 
-// { dg-error "period must be positive" "" { target *-*-* } 303 }
+// { dg-error "period must be positive" "" { target *-*-* } 321 }
diff --git a/libstdc++-v3/testsuite/20_util/time_point/cons/81468.cc b/libstdc++-v3/testsuite/20_util/time_point/cons/81468.cc
new file mode 100644
index 00000000000..30d1c4a5ac7
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/time_point/cons/81468.cc
@@ -0,0 +1,34 @@ 
+// Copyright (C) 2017 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do compile { target c++11 } }
+
+#include <chrono>
+#include <type_traits>
+
+using namespace std;
+using namespace std::chrono;
+
+template <class Duration>
+    using sys_time = time_point<system_clock, Duration>;
+
+static_assert(is_constructible<sys_time<milliseconds>, sys_time<seconds>>{},
+    "Can construct time_point from one with lower precision duration");
+
+// PR libstdc++/81468 - DR 1177
+static_assert(!is_constructible<sys_time<seconds>, sys_time<milliseconds>>{},
+    "Cannot construct time_point from one with higher precision duration");