function: Also error for read-only var in positional arg

We have this hack where any positional arguments are taken as argument
names if "--argument-names" is given, and that didn't check for
read-only variables.

Fixes #11295

(cherry picked from commit d203ee4d53)

src/builtins/function.rs

@@ -92,6 +92,14 @@ fn parse_cmd_opts(
                 // A positional argument we got because we use RETURN_IN_ORDER.
                 let woptarg = w.woptarg.unwrap().to_owned();
                 if handling_named_arguments {
+                    if is_read_only(&woptarg) {
+                        streams.err.append(wgettext_fmt!(
+                            "%ls: variable '%ls' is read-only\n",
+                            cmd,
+                            woptarg
+                        ));
+                        return STATUS_INVALID_ARGS;
+                    }
                     opts.named_arguments.push(woptarg);
                 } else {
                     streams.err.append(wgettext_fmt!(

tests/checks/function.fish

@@ -183,6 +183,11 @@ function foo --argument-names status; end
 echo status $status
 # CHECK: status 2
 
+function foo --argument-names foo status; end
+# CHECKERR: {{.*}}function.fish (line {{\d+}}): function: variable 'status' is read-only
+# CHECKERR: function foo --argument-names foo status; end
+# CHECKERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
+
 functions -q foo
 echo exists $status
 # CHECK: exists 1