testsuite: run C++11 tests in C++11 mode

A recent change missed updating constexpr-mutable3.C because it wasn't run
in C++11 mode even though it checks the behavior for { target c++11_only }.

gcc/testsuite/ChangeLog:

	* lib/g++-dg.exp (g++-dg-runtest): Check for c++11_only.

1 files changed, 6 insertions, 3 deletions

diff --git a/gcc/testsuite/lib/g++-dg.exp b/gcc/testsuite/lib/g++-dg.exp
index 046d631..142c52c 100644
--- a/gcc/testsuite/lib/g++-dg.exp
+++ b/gcc/testsuite/lib/g++-dg.exp
@@ -55,13 +55,16 @@ proc g++-dg-runtest { testcases flags default-extra-flags } {
 	    } else {
 		# If the test requires a newer C++ version than which
 		# is tested by default, use that C++ version for that
-		# single test.  This should be updated or commented
-		# out whenever the default std_list is updated or newer
-		# C++ effective target is added.
+		# single test.  Or if a test checks behavior specifically for
+		# one C++ version, include that version in the default list.
+		# These should be adjusted whenever the default std_list is
+		# updated or newer C++ effective target is added.
 		if [search_for $test "\{ dg-do * \{ target c++23"] {
 		    set std_list { 23 26 }
 		} elseif [search_for $test "\{ dg-do * \{ target c++26"] {
 		    set std_list { 26 }
+		} elseif [search_for $test "c++11_only"] {
+		    set std_list { 98 11 14 20 }
 		} else {
 		    set std_list { 98 14 17 20 }
 		}