toolchain-funcs.eclass: Add tc-get-cxx-stdlib() to get C++ stdlib

Add a new tc-get-cxx-stdlib() that attempts to get the C++ stdlib
variant used by the current C++ compiler.  Currently it supports libc++
and libstdc++ (GCC's stdlib).

Signed-off-by: Michał Górny <mgorny@gentoo.org>

2 files changed, 58 insertions, 0 deletions

diff --git a/eclass/tests/toolchain-funcs.sh b/eclass/tests/toolchain-funcs.sh
index 56379b10cded..5a35a44ce018 100755
--- a/eclass/tests/toolchain-funcs.sh
+++ b/eclass/tests/toolchain-funcs.sh
@@ -198,4 +198,26 @@ for compiler in gcc clang not-really-a-compiler; do
 	fi
 done
 
+if type -P gcc &>/dev/null; then
+	tbegin "tc-get-cxx-stdlib (gcc)"
+	[[ $(CXX=g++ tc-get-cxx-stdlib) == libstdc++ ]]
+	tend $?
+fi
+
+if type -P clang &>/dev/null; then
+	for stdlib in libc++ libstdc++; do
+		if clang++ -stdlib=${stdlib} -x c++ -E -P - &>/dev/null \
+			<<<'#include <ciso646>'
+		then
+			tbegin "tc-get-cxx-stdlib (clang, ${stdlib})"
+			[[ $(CXX=clang++ CXXFLAGS="-stdlib=${stdlib}" tc-get-cxx-stdlib) == ${stdlib} ]]
+			tend $?
+		fi
+	done
+
+	tbegin "tc-get-cxx-stdlib (clang, invalid)"
+	! CXX=clang++ CXXFLAGS="-stdlib=invalid" tc-get-cxx-stdlib
+	tend $?
+fi
+
 texit
diff --git a/eclass/toolchain-funcs.eclass b/eclass/toolchain-funcs.eclass
index 48bf11606c4a..92494158201e 100644
--- a/eclass/toolchain-funcs.eclass
+++ b/eclass/toolchain-funcs.eclass
@@ -1173,4 +1173,40 @@ gen_usr_ldscript() {
 	done
 }
 
+# @FUNCTION: tc-get-cxx-stdlib
+# @DESCRIPTION:
+# Attempt to identify the C++ standard library used by the compiler.
+# If the library is identified, the function returns 0 and prints one
+# of the following:
+#
+# - ``libc++`` for ``sys-libs/libcxx``
+# - ``libstdc++`` for ``sys-devel/gcc``'s libstdc++
+#
+# If the library is not recognized, the function returns 1.
+tc-get-cxx-stdlib() {
+	local code='#include <ciso646>
+
+#if defined(_LIBCPP_VERSION)
+	HAVE_LIBCXX
+#elif defined(__GLIBCXX__)
+	HAVE_LIBSTDCPP
+#endif
+'
+	local res=$(
+		$(tc-getCXX) ${CXXFLAGS} ${CPPFLAGS} -x c++ -E -P - \
+			<<<"${code}" 2>/dev/null
+	)
+
+	case ${res} in
+		*HAVE_LIBCXX*)
+			echo libc++;;
+		*HAVE_LIBSTDCPP*)
+			echo libstdc++;;
+		*)
+			return 1;;
+	esac
+
+	return 0
+}
+
 fi