[Fix] allow `nvm unalias x` when `x` is a default alias, but shadowed

Fixes #2122.

nvm.sh

@@ -3372,14 +3372,24 @@ nvm() {
       local NVM_NODE_PREFIX
       NVM_IOJS_PREFIX="$(nvm_iojs_prefix)"
       NVM_NODE_PREFIX="$(nvm_node_prefix)"
-      case "$1" in
-        "stable" | "unstable" | "${NVM_IOJS_PREFIX}" | "${NVM_NODE_PREFIX}" | "system")
-          nvm_err "${1-} is a default (built-in) alias and cannot be deleted."
-          return 1
-        ;;
-      esac
+      local NVM_ALIAS_EXISTS
+      NVM_ALIAS_EXISTS=0
+      if [ -f "${NVM_ALIAS_DIR}/${1-}" ]; then
+        NVM_ALIAS_EXISTS=1
+      fi
+
+      if [ $NVM_ALIAS_EXISTS -eq 0 ]; then
+        case "$1" in
+          "stable" | "unstable" | "${NVM_IOJS_PREFIX}" | "${NVM_NODE_PREFIX}" | "system")
+            nvm_err "${1-} is a default (built-in) alias and cannot be deleted."
+            return 1
+          ;;
+        esac
+
+        nvm_err "Alias ${1-} doesn't exist!"
+        return
+      fi
 
-      [ ! -f "${NVM_ALIAS_DIR}/${1-}" ] && nvm_err "Alias ${1-} doesn't exist!" && return
       local NVM_ALIAS_ORIGINAL
       NVM_ALIAS_ORIGINAL="$(nvm_alias "${1}")"
       command rm -f "${NVM_ALIAS_DIR}/${1}"

test/fast/Aliases/"nvm unalias" should accept aliases when they shadow a built-in alias

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+\. ../../../nvm.sh
+
+die () { echo "$@" ; exit 1; }
+
+OUTPUT="$(nvm unalias node 2>&1)"
+EXPECTED_OUTPUT="node is a default (built-in) alias and cannot be deleted."
+[ "$OUTPUT" = "$EXPECTED_OUTPUT" ] || die "trying to remove a built-in alias should fail with '$EXPECTED_OUTPUT', got '$OUTPUT'"
+
+nvm alias node stable || die '`nvm alias node stable` failed'
+
+nvm unalias node || die '`nvm unalias node` failed'
+
+OUTPUT="$(nvm unalias node 2>&1)"
+EXPECTED_OUTPUT="node is a default (built-in) alias and cannot be deleted."
+[ "$OUTPUT" = "$EXPECTED_OUTPUT" ] || die "trying to remove a built-in alias should fail with '$EXPECTED_OUTPUT', got '$OUTPUT'"