Hi!

On Wed, Oct 19, 2022 at 09:10:48AM -0400, Jason Merrill wrote:
> > The screw-up on my side with libstdc++ testing (tested normally rather
> > than in C++23 mode) makes me wonder if we couldn't tweak the default
> > testing.
> > Dunno what libstdc++ testing normally does (just C++17?), make check-g++
> > tests by default { 98, 14, 17, 20 } (and I regularly use
> > GXX_TESTSUITE_STDS=98,11,14,17,20,2b in environment but that doesn't
> > cover libstdc++ I guess).
> > When adding tests for upcoming C++ version, one always has a dilemma
> > whether to use explicit // { dg-options "-std=c++2b" }
> > or -std=gnu++2b and similar, then the test works in all modes, but it might
> > be forgotten later on to be converted into // { dg-do whatever { target c++23 } }
> > test so that when 23 is tested by default and say 26 or 29 appears too,
> > we test it also in those modes, or just go with
> > // { dg-do whatever { target c++23 } }
> > which has the disadvantage that it is skipped when testing by default and
> > one only tests it if he asks for the newer version.
> > 
> > I wonder if we couldn't for the default testing (when one doesn't
> > specify GXX_TESTSUITE_STDS or uses make check-c++-all and similar)
> > improve things a little bit by automatically treat those
> > // { dg-do whatever { target c++23 } }
> > tests as // { dg-options "-std=c++2b" }.
> 
> That would be great.

Here is a patch that implements it.
On make check-g++ RUNTESTFLAGS=dg.exp=cpp23/*.C
the vanilla vs. patched gcc difference is:
                === g++ Summary ===

# of expected passes            1743
# of expected failures          2
# of unsupported tests          224
vs.
                === g++ Summary ===

# of expected passes            1829
# of expected failures          2
# of unsupported tests          112

(g++.sum details attached).

2022-10-19  Jakub Jelinek  <jakub@redhat.com>

	* lib/g++-dg.exp (g++-dg-runtest): When using defaulted
	std_list, if test has { dg-do * { target c++23 } } directive,
	use { 23 } with which the test will run instead of { 98 14 17 20 }
	which would make it UNSUPPORTED in all cases.

--- gcc/testsuite/lib/g++-dg.exp.jj	2022-03-30 09:11:53.306103969 +0200
+++ gcc/testsuite/lib/g++-dg.exp	2022-10-19 17:10:51.574866364 +0200
@@ -53,7 +53,16 @@ proc g++-dg-runtest { testcases flags de
 	    if { [llength $gpp_std_list] > 0 } {
 		set std_list $gpp_std_list
 	    } else {
-		set std_list { 98 14 17 20 }
+		# 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.
+		if [search_for $test "{ dg-do * { target c++23 } }"] {
+		    set std_list { 23 }
+		} else {
+		    set std_list { 98 14 17 20 }
+		}
 	    }
 	    set option_list { }
 	    foreach x $std_list {


	Jakub