Always heightenize file descriptors

Fixes #12618

Closes #12681

tests/checks/fds.fish

@@ -1,4 +1,4 @@
-# RUN: %fish -C "set helper %fish_test_helper" %s
+# RUN: fish=%fish %fish -C "set helper %fish_test_helper" %s
 #REQUIRES: command -v %fish_test_helper
 
 # Check that we don't leave stray FDs.
@@ -89,3 +89,39 @@ $helper print_fds 19</dev/null
 
 $helper print_fds 20</dev/null
 # CHECK: 0 1 2 20
+
+# Check heightenization of the redirection file descriptors
+#
+# If heightenization does NOT work, ">$tmpfile" creates a temporary fd on a fd n
+# (n being the first free descriptor) during parsing, then "n>&1" redirects fd n
+# to fd 1 (still stdout at this stage), lastly, ">$tmpfile" redirects fd 1 is
+# redirected to the "temporary" fd n (which is now stdout instead of $tmpfile).
+#
+# If heightenization does work, ">$tmpfile" creates a temporary fd on 10+,
+# then fd n is redirected to fd 1 (still stdout), the fd 1 is redirect to
+# the "temporary" fd 10+ (still $tmpfile)
+#
+# Since we don't know what `n` will be, we need to redirect all the descriptors
+# reserved for the user
+set -l tmpfile (mktemp)
+printf 'stdout: %s\n' "$($fish -c "$helper print_fds 3>&1 4>&1 5>&1 6>&1 7>&1 8>&1 9>&1 >$tmpfile")"
+printf "tmp file: %s\n" $(cat $tmpfile)
+# CHECK: stdout:
+# CHECK: tmp file: 0 1 2 3 4 5 6 7 8 9
+
+# Check heightenization of the file descriptors when sourcing
+echo '
+path filter /proc/$fish_pid/fd/{3,4,5,6,7,8,9}
+' >"$tmpfile"
+echo start source
+$fish -c "source '$tmpfile'"
+echo end source
+# CHECK: start source
+# CHECK: end source
+
+# Check heightenization of descriptors for script files passed to fish
+echo start fish
+$fish "$tmpfile"
+echo end fish
+# CHECK: start fish
+# CHECK: end fish