Merge 5f7d419458beeecc1898a0181c318cffe2601dc8 into 6b70c40f151ca051d403453bf019e1707b33bd5b

Fixes #3447

nvm.sh

@@ -196,6 +196,9 @@ nvm_install_latest_npm() {
   nvm_echo 'Attempting to upgrade to the latest working version of npm...'
   local NODE_VERSION
   NODE_VERSION="$(nvm_strip_iojs_prefix "$(nvm_ls_current)")"
+  local NPM_VERSION
+  NPM_VERSION="$(npm --version 2>/dev/null)"
+
   if [ "${NODE_VERSION}" = 'system' ]; then
     NODE_VERSION="$(node --version)"
   elif [ "${NODE_VERSION}" = 'none' ]; then
@@ -206,8 +209,6 @@ nvm_install_latest_npm() {
     nvm_err 'Unable to obtain node version.'
     return 1
   fi
-  local NPM_VERSION
-  NPM_VERSION="$(npm --version 2>/dev/null)"
   if [ -z "${NPM_VERSION}" ]; then
     nvm_err 'Unable to obtain npm version.'
     return 2
@@ -4529,7 +4530,11 @@ nvm_supports_xz() {
   if [ "_${NVM_OS}" = '_darwin' ]; then
     local MACOS_VERSION
     MACOS_VERSION="$(sw_vers -productVersion)"
-    if nvm_version_greater "10.9.0" "${MACOS_VERSION}"; then
+    if tar --version | command grep -q GNU && ! command which xz >/dev/null 2>&1; then
+      # On macOS with GNU tar in use, and no xv on the path, xv-compressed
+      # tarballs aren't supported
+      return 1
+    elif nvm_version_greater "10.9.0" "${MACOS_VERSION}"; then
       # macOS 10.8 and earlier doesn't support extracting xz-compressed tarballs with tar
       return 1
     fi