improve scoping with regard to function variables existing

src/args.rs

@@ -127,6 +127,19 @@ impl CallArgs {
         }
     }
 
+    pub fn get(
+        &mut self,
+        position: usize,
+        name: String,
+        scope: &Scope,
+        super_selector: &Selector,
+    ) -> Option<SassResult<Spanned<Value>>> {
+        match self.get_named(name, scope, super_selector) {
+            Some(v) => return Some(v),
+            None => return self.get_positional(position, scope, super_selector),
+        }
+    }
+
     pub fn get_variadic(
         self,
         scope: &Scope,

src/atrule/function.rs

@@ -69,11 +69,11 @@ impl Function {
         super_selector: &Selector,
     ) -> SassResult<()> {
         let mut scope = scope.clone();
-        for (idx, arg) in self.args.0.iter().enumerate() {
+        for (idx, arg) in self.args.0.iter_mut().enumerate() {
             if arg.is_variadic {
                 let span = args.span();
                 let arg_list = Value::ArgList(args.get_variadic(&scope, super_selector)?);
-                scope.insert_var(
+                self.scope.insert_var(
                     &arg.name,
                     Spanned {
                         node: arg_list,
@@ -82,27 +82,24 @@ impl Function {
                 )?;
                 break;
             }
-            let val = match args.get_positional(idx, &scope, super_selector) {
+            let val = match args.get(idx, arg.name.clone(), &scope, super_selector) {
                 Some(v) => v?,
-                None => match args.get_named(arg.name.clone(), &scope, super_selector) {
-                    Some(v) => v?,
-                    None => match &arg.default {
-                        Some(v) => Value::from_tokens(
-                            &mut v.iter().cloned().peekmore(),
-                            &scope,
-                            super_selector,
-                        )?,
-                        None => {
-                            return Err(
-                                (format!("Missing argument ${}.", &arg.name), args.span()).into()
-                            )
-                        }
-                    },
+                None => match arg.default.as_mut() {
+                    Some(v) => Value::from_tokens(
+                        &mut std::mem::take(v).into_iter().peekmore(),
+                        &scope,
+                        super_selector,
+                    )?,
+                    None => {
+                        return Err(
+                            (format!("Missing argument ${}.", &arg.name), args.span()).into()
+                        )
+                    }
                 },
             };
-            scope.insert_var(&arg.name, val)?;
+            scope.insert_var(&arg.name, val.clone())?;
+            self.scope.insert_var(&arg.name, val)?;
         }
-        self.scope.extend(scope);
         Ok(())
     }
 

tests/scope.rs

@@ -23,3 +23,20 @@ test!(
     "$y: a;\n@mixin foo {\n  $y: b !global;\n}\na {\n  @include foo;\n  color: $y;\n}\n",
     "a {\n  color: b;\n}\n"
 );
+test!(
+    local_variable_exists_in_fn_mixin_scope,
+    "@function exists-fn($name) {
+        @return variable-exists($name);
+    }
+
+    @mixin exists-mixin($name) {
+        color: variable-exists($name);
+    }
+
+    a {
+        $x: foo;
+        @include exists-mixin(x);
+        color: exists-fn(x);
+    }",
+    "a {\n  color: false;\n  color: false;\n}\n"
+);