[gentoo-dev] [PATCH 2/3] flag-o-matic.eclass: test-flag-PROG, ignore unused args in clang

["Michał Górny" <mgorny@gentoo.org>]

By default, clang considers unused arguments as error when -Werror is
used. Since flag tests are performed without linking, this causes all
tests for linker flags to fail inadvertently and all those flags
are stripped as a result.

While the correctness of passing unused flags is doubtful, silently
stripping them in a few random packages is certainly not the solution
to the problem, and also makes the results differ between gcc and clang.
To account for that, use clang's -Qunused-arguments option to silence
unused argument warnings.

To avoid wasting time on testing the compiler, just try passing
-Qunused-arguments every time a flag check fails. If clang is not used,
the additional call will fail just the same as the previous one (either
because of the original flag or because of -Qunused-arguments), so
the result will be the same.
---
 eclass/flag-o-matic.eclass   | 9 ++++++++-
 eclass/tests/flag-o-matic.sh | 5 +++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/eclass/flag-o-matic.eclass b/eclass/flag-o-matic.eclass
index 0393a30b74c3..79866e04a483 100644
--- a/eclass/flag-o-matic.eclass
+++ b/eclass/flag-o-matic.eclass
@@ -441,7 +441,14 @@ test-flag-PROG() {
 		cmdline+=( "${flag}" -c -o /dev/null /dev/null )
 	fi
 
-	"${cmdline[@]}" </dev/null &>/dev/null
+	if ! "${cmdline[@]}" </dev/null &>/dev/null; then
+		# -Werror makes clang bail out on unused arguments as well;
+		# try to add -Qunused-arguments to work-around that
+		# other compilers don't support it but then, it's failure like
+		# any other
+		cmdline+=( -Qunused-arguments )
+		"${cmdline[@]}" </dev/null &>/dev/null
+	fi
 }
 
 # @FUNCTION: test-flag-CC
diff --git a/eclass/tests/flag-o-matic.sh b/eclass/tests/flag-o-matic.sh
index 92c68b82c3c9..5e7ee354bf33 100755
--- a/eclass/tests/flag-o-matic.sh
+++ b/eclass/tests/flag-o-matic.sh
@@ -143,6 +143,11 @@ tbegin "test-flags-CC (gcc-valid but clang-invalid flags)"
 out=$(CC=clang test-flags-CC -finline-limit=1200)
 [[ $? -ne 0 && -z ${out} ]]
 ftend
+
+tbegin "test-flags-CC (unused flags w/clang)"
+out=$(CC=clang test-flags-CC -Wl,-O1)
+[[ $? -eq 0 && ${out} == "-Wl,-O1" ]]
+ftend
 fi
 
 texit
-- 
2.14.0