function: Check if --argument-names gets a valid variable name

These were accepted but then ineffective because the only way these
are used is to set a variable.

src/builtins/function.rs

@@ -320,6 +320,15 @@ pub fn function(
         })
         .collect();
 
+    for named in &opts.named_arguments {
+        if !valid_var_name(&named) {
+            streams
+                .err
+                .append(wgettext_fmt!(BUILTIN_ERR_VARNAME, cmd, named));
+            return STATUS_INVALID_ARGS;
+        }
+    }
+
     // We have what we need to actually define the function.
     let props = function::FunctionProperties {
         func_node,

tests/checks/function.fish

@@ -167,4 +167,10 @@ function foo -p bar; end
 # CHECKERR: function foo -p bar; end
 # CHECKERR: ^~~~~~~~~~~~~~~~~~~^
 
+function foo --argument-names "banana pajama"; end
+# CHECKERR: {{.*}}function.fish (line {{\d+}}): function: banana pajama: invalid variable name. See `help identifiers`
+# CHECKERR: function foo --argument-names "banana pajama"; end
+# CHECKERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
+
+
 exit 0