testsuite: Fix up g++.dg/cpp2a/concepts-lambda3.C [PR108972]

testsuite: Fix up g++.dg/cpp2a/concepts-lambda3.C [PR108972]

On Fri, Mar 10, 2023 at 01:49:38PM -0500, Jason Merrill via Gcc-patches wrote:
> gcc/testsuite/ChangeLog:
>
>       * g++.dg/cpp2a/concepts-lambda3.C: Run at lower std levels,
>       but expect errors.

I'm seeing
+UNRESOLVED: g++.dg/cpp2a/concepts-lambda3.C  -std=c++11 compilation failed to produce executable
+UNRESOLVED: g++.dg/cpp2a/concepts-lambda3.C  -std=c++14 compilation failed to produce executable
+UNRESOLVED: g++.dg/cpp2a/concepts-lambda3.C  -std=c++17 compilation failed to produce executable
+UNRESOLVED: g++.dg/cpp2a/concepts-lambda3.C  -std=c++98 compilation failed to produce executable
with this change, and if I test with
GXX_TESTSUITE_STDS=98,11,14,17,20,2b make check-g++ -k RUNTESTFLAGS="--target_board=unix\{-m32,-m64,-m64/-fconcepts\} dg.exp=concepts-lambda3.C"
I see even FAILs for the -fconcepts case, so apparently even -std=c++17
-fconcepts isn't enough to make it compile without errors.

The following patch will expect errors for all of c++17_down and will
make the test dg-do compile for that case too, such that the UNRESOLVED
stuff is gone.

2023-03-14  Jakub Jelinek  <jakub@redhat.com>

	PR c++/108972
	PR testsuite/109129
	* g++.dg/cpp2a/concepts-lambda3.C: Use dg-do run only for c++20,
	for c++17_down dg-do compile.  Expect dg-excess-errors for c++17_down
	rather than ! concepts.