commit 8c9891a3828dc6a4c91998b2437ef9fbf8659163
Author: Jason Merrill <jason@redhat.com>
Date:   Fri May 8 17:01:37 2015 -0500

    gcc/c-family/
    	* c-common.h (enum cxx_dialect): Add cxx_unset.
    	* c-common.c (cxx_dialect): Initialize to cxx_unset.
    	* c-opts.c (c_common_post_options): Set C++ dialect to C++98 if unset.
    gcc/testsuite/
    	* lib/target-supports.exp (cxx_default): New global.
    	(check_effective_target_c++11_only)
    	(check_effective_target_c++14_only)
    	(check_effective_target_c++98_only)
    	(check_effective_target_c++1z_only): Check it.

diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
index 378f237..93b3060 100644
--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -256,9 +256,9 @@ const char *constant_string_class_name;
 
 int flag_use_repository;
 
-/* The C++ dialect being used. C++98 is the default.  */
+/* The C++ dialect being used.  Default set in c_common_post_options.  */
 
-enum cxx_dialect cxx_dialect = cxx98;
+enum cxx_dialect cxx_dialect = cxx_unset;
 
 /* Maximum template instantiation depth.  This limit exists to limit the
    time it takes to notice excessively recursive template instantiations.
diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
index 603d3f0..5c1fa7b 100644
--- a/gcc/c-family/c-common.h
+++ b/gcc/c-family/c-common.h
@@ -671,6 +671,7 @@ extern int flag_use_repository;
 /* The supported C++ dialects.  */
 
 enum cxx_dialect {
+  cxx_unset,
   /* C++98 with TC1  */
   cxx98,
   cxx03 = cxx98,
diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
index 8b17674..bd99871 100644
--- a/gcc/c-family/c-opts.c
+++ b/gcc/c-family/c-opts.c
@@ -896,6 +896,10 @@ c_common_post_options (const char **pfilename)
   if (flag_abi_version == 0)
     flag_abi_version = 8;
 
+  /* Set C++ standard to C++98 if not specified on the command line.  */
+  if (c_dialect_cxx () && cxx_dialect == cxx_unset)
+    set_std_cxx98 (/*ISO*/false);
+
   if (cxx_dialect >= cxx11)
     {
       /* If we're allowing C++0x constructs, don't warn about C++98
diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
index d68b48b..3728927 100644
--- a/gcc/testsuite/lib/target-supports.exp
+++ b/gcc/testsuite/lib/target-supports.exp
@@ -5892,15 +5892,23 @@ proc check_effective_target_c++ { } {
  return 0
 }
 
+set cxx_default "c++98"
 # Check whether the current active language standard supports the features
-# of C++11/C++14 by checking for the presence of one of the -std
-# flags.  This assumes that the default for the compiler is C++98, and that
+# of C++11/C++14 by checking for the presence of one of the -std flags.
+# This assumes that the default for the compiler is $cxx_default, and that
 # there will never be multiple -std= arguments on the command line.
 proc check_effective_target_c++11_only { } {
+    global cxx_default
     if ![check_effective_target_c++] {
 	return 0
     }
-    return [check-flags { { } { } { -std=c++0x -std=gnu++0x -std=c++11 -std=gnu++11 } }]
+    if [check-flags { { } { } { -std=c++0x -std=gnu++0x -std=c++11 -std=gnu++11 } }] {
+	return 1
+    }
+    if { $cxx_default == "c++11" && [check-flags { { } { } { } { -std=* } }] } {
+	return 1
+    }
+    return 0
 }
 proc check_effective_target_c++11 { } {
     if [check_effective_target_c++11_only] {
@@ -5912,14 +5920,21 @@ proc check_effective_target_c++11_down { } {
     if ![check_effective_target_c++] {
 	return 0
     }
-    return ![check_effective_target_c++14]
+    return [expr ![check_effective_target_c++14] ]
 }
 
 proc check_effective_target_c++14_only { } {
+    global cxx_default
     if ![check_effective_target_c++] {
 	return 0
     }
-    return [check-flags { { } { } { -std=c++14 -std=gnu++14 -std=c++14 -std=gnu++14 } }]
+    if [check-flags { { } { } { -std=c++14 -std=gnu++14 -std=c++14 -std=gnu++14 } }] {
+	return 1
+    }
+    if { $cxx_default == "c++14" && [check-flags { { } { } { } { -std=* } }] } {
+	return 1
+    }
+    return 0
 }
 
 proc check_effective_target_c++14 { } {
@@ -5932,21 +5947,35 @@ proc check_effective_target_c++14_down { } {
     if ![check_effective_target_c++] {
 	return 0
     }
-    return ![check_effective_target_c++1z]
+    return [expr ![check_effective_target_c++1z] ]
 }
 
 proc check_effective_target_c++98_only { } {
+    global cxx_default
     if ![check_effective_target_c++] {
 	return 0
     }
-    return ![check_effective_target_c++11]
+    if [check-flags { { } { } { -std=c++98 -std=gnu++98 -std=c++03 -std=gnu++03 } }] {
+	return 1
+    }
+    if { $cxx_default == "c++98" && [check-flags { { } { } { } { -std=* } }] } {
+	return 1
+    }
+    return 0
 }
 
 proc check_effective_target_c++1z_only { } {
+    global cxx_default
     if ![check_effective_target_c++] {
 	return 0
     }
-    return [check-flags { { } { } { -std=c++1z -std=gnu++1z } }]
+    if [check-flags { { } { } { -std=c++17 -std=gnu++17 -std=c++1z -std=gnu++1z } }] {
+	return 1
+    }
+    if { $cxx_default == "c++17" && [check-flags { { } { } { } { -std=* } }] } {
+	return 1
+    }
+    return 0
 }
 proc check_effective_target_c++1z { } {
     return [check_effective_target_c++1z_only]