diff --git a/llvm-runtimes/libcxx/libcxx-20.1.8-r1.ebuild b/llvm-runtimes/libcxx/libcxx-20.1.8-r1.ebuild
index e0e4478f8d0d2..40f9d39127684 100644
--- a/llvm-runtimes/libcxx/libcxx-20.1.8-r1.ebuild
+++ b/llvm-runtimes/libcxx/libcxx-20.1.8-r1.ebuild
@@ -145,6 +145,12 @@ multilib_src_configure() {
 		# this is broken with standalone builds, and also meaningless
 		-DLIBCXXABI_USE_LLVM_UNWINDER=OFF
 	)
+	if ! has_version -b sys-devel/gcc; then
+		# Since this package is merged before llvm-runtimes/clang-stdlib-config,
+		# clang will attempt to use libstdc++ for the C++ compiler check, and will
+		# fail if it is missing.
+		mycmakeargs+=( -DCMAKE_CXX_COMPILER_WORKS=1 )
+	fi
 	if is_crosspkg; then
 		# Needed to target built libc headers
 		local -x CFLAGS="${CFLAGS} -isystem ${ESYSROOT}/usr/${CTARGET}/usr/include"
diff --git a/llvm-runtimes/libcxx/libcxx-22.1.0_rc1.ebuild b/llvm-runtimes/libcxx/libcxx-22.1.0_rc1.ebuild
index 7d224001ae1b6..670bccda0c24a 100644
--- a/llvm-runtimes/libcxx/libcxx-22.1.0_rc1.ebuild
+++ b/llvm-runtimes/libcxx/libcxx-22.1.0_rc1.ebuild
@@ -144,6 +144,12 @@ multilib_src_configure() {
 		# this is broken with standalone builds, and also meaningless
 		-DLIBCXXABI_USE_LLVM_UNWINDER=OFF
 	)
+	if ! has_version -b sys-devel/gcc; then
+		# Since this package is merged before llvm-runtimes/clang-stdlib-config,
+		# clang will attempt to use libstdc++ for the C++ compiler check, and will
+		# fail if it is missing.
+		mycmakeargs+=( -DCMAKE_CXX_COMPILER_WORKS=1 )
+	fi
 	if is_crosspkg; then
 		# Needed to target built libc headers
 		local -x CFLAGS="${CFLAGS} -isystem ${ESYSROOT}/usr/${CTARGET}/usr/include"