--- libcxx-11.0.0.mainline.ebuild	2020-11-30 15:51:51.000000000 -0600
+++ libcxx-11.0.0.ebuild	2020-12-01 23:17:06.000000000 -0600
@@ -5,6 +5,7 @@
 
 CMAKE_ECLASS=cmake
 PYTHON_COMPAT=( python3_{6..9} )
+# prefix change might need flag-o-matic
 inherit cmake-multilib llvm llvm.org python-any-r1 toolchain-funcs
 
 DESCRIPTION="New implementation of the C++ standard library, targeting C++11"
@@ -14,9 +15,10 @@
 
 LICENSE="Apache-2.0-with-LLVM-exceptions || ( UoI-NCSA MIT )"
 SLOT="0"
-KEYWORDS="amd64 arm arm64 x86"
-IUSE="elibc_glibc elibc_musl +libcxxabi +libunwind +static-libs test"
-REQUIRED_USE="libunwind? ( libcxxabi )"
+KEYWORDS="amd64 arm arm64 x86 ~x64-macos"
+IUSE="elibc_Darwin elibc_glibc elibc_musl +libcxxabi +libunwind +static-libs test"
+REQUIRED_USE="libunwind? ( libcxxabi )
+	elibc_Darwin? ( libcxxabi )"
 RESTRICT="!test? ( test )"
 
 RDEPEND="
@@ -58,6 +60,11 @@
 	eapply "${FILESDIR}/${PN}-3.9-cmake-link-flags.patch"
 
 	llvm.org_src_prepare
+
+	# eprefixify static path references to libc++abi for symbol re-export to
+	# avoid linking against it twice in both /usr/lib and ${EPREFIX}/usr/lib
+	local lcpa=/usr/lib/libc++abi.dylib
+	sed -i -e "s,${lcpa},${EPREFIX}${lcpa},g" lib/CMakeLists.txt
 }
 
 test_compiler() {
@@ -100,6 +107,8 @@
 			fi
 		fi
 	fi
+	# Darwin has libunwind in its system
+	[[ ${CHOST} == *-darwin* ]] && want_gcc_s=OFF
 
 	# bootstrap: cmake is unhappy if compiler can't link to stdlib
 	local nolib_flags=( -nodefaultlibs -lc )
@@ -136,6 +145,12 @@
 			-DPython3_EXECUTABLE="${PYTHON}"
 		)
 	fi
+
+	# prefix change needed?
+	# make sure we build multilib on OSX, because llvm insists on
+	# building multilib too
+	#[[ ${CHOST} == *86*-darwin* ]] && append-flags -arch i386 -arch x86_64
+
 	cmake_src_configure
 }
 
@@ -188,8 +203,10 @@
 
 multilib_src_install() {
 	cmake_src_install
-	gen_shared_ldscript
-	use static-libs && gen_static_ldscript
+	if [[ ${CHOST} != *-darwin* ]] ; then
+		gen_shared_ldscript
+		use static-libs && gen_static_ldscript
+	fi
 }
 
 pkg_postinst() {