diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 4242c9869ff..85c286a4a15 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2019-06-24  Jonathan Wakely  <jwakely@redhat.com>
+
+	* ginclude/float.h (FLT_DECIMAL_DIG, DBL_DECIMAL_DIG, LDBL_DECIMAL_DIG)
+	(FLT_HAS_SUBNORM, DBL_HAS_SUBNORM, LDBL_HAS_SUBNORM, FLT_TRUE_MIN)
+	(DBL_TRUE_MIN, LDBL_TRUE_MIN): Also define for C++17.
+
 2019-06-23  Iain Sandoe  <iain@sandoe.co.uk>
 
 	* config/rs6000/darwin.h: Handle GCC target pragma.
diff --git a/gcc/ginclude/float.h b/gcc/ginclude/float.h
index 9ebae057d34..4767d7b9dfb 100644
--- a/gcc/ginclude/float.h
+++ b/gcc/ginclude/float.h
@@ -210,7 +210,8 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 
 #endif /* C99 */
 
-#if defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+#if (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) \
+     || (defined (__cplusplus) && __cplusplus >= 201703L)
 /* Versions of DECIMAL_DIG for each floating-point type.  */
 #undef FLT_DECIMAL_DIG
 #undef DBL_DECIMAL_DIG
diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index ac98c0d02e7..2acb1508978 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,7 @@
+2019-06-24  Jonathan Wakely  <jwakely@redhat.com>
+
+	* testsuite/18_support/headers/cfloat/values_c++17.cc: New test.
+
 2019-06-20  Jonathan Wakely  <jwakely@redhat.com>
 
 	* acinclude.m4 (GLIBCXX_ENABLE_DEBUG): Only do debug build for final
diff --git a/libstdc++-v3/testsuite/18_support/headers/cfloat/values_c++17.cc b/libstdc++-v3/testsuite/18_support/headers/cfloat/values_c++17.cc
new file mode 100644
index 00000000000..f3efd204d17
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/headers/cfloat/values_c++17.cc
@@ -0,0 +1,37 @@
+// { dg-options "-std=gnu++17" }
+// { dg-do compile { target c++17 } }
+
+// Copyright (C) 2019 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/>.
+
+#include <cfloat>
+#include "values.cc"
+
+namespace gnu17
+{
+  double d1 = DBL_DECIMAL_DIG;
+  double d2 = DBL_HAS_SUBNORM;
+  double d3 = DBL_TRUE_MIN;
+
+  float f1 = FLT_DECIMAL_DIG;
+  float f2 = FLT_HAS_SUBNORM;
+  float f3 = FLT_TRUE_MIN;
+
+  long double ld1 = LDBL_DECIMAL_DIG;
+  long double ld2 = LDBL_HAS_SUBNORM;
+  long double ld3 = LDBL_TRUE_MIN;
+}