diff --git a/compiler/erg_compiler/context/initialize/mod.rs b/compiler/erg_compiler/context/initialize/mod.rs
index 24f2f1c0..2a83992e 100644
--- a/compiler/erg_compiler/context/initialize/mod.rs
+++ b/compiler/erg_compiler/context/initialize/mod.rs
@@ -2035,7 +2035,8 @@ impl Context {
             vec![kw("iterable", poly("Iterable", vec![ty_tp(T.clone())]))],
             None,
             array_t(T.clone(), TyParam::erased(Nat)),
-        );
+        )
+        .quantify();
         let t_str = nd_func(vec![kw("object", Obj)], None, Str);
         let A = mono_q("A", Constraint::Uninited);
         let A = mono_q("A", subtypeof(poly("Add", vec![ty_tp(A)])));
diff --git a/compiler/erg_compiler/error.rs b/compiler/erg_compiler/error.rs
index 5875ef41..5efb5f5c 100644
--- a/compiler/erg_compiler/error.rs
+++ b/compiler/erg_compiler/error.rs
@@ -1995,64 +1995,6 @@ impl LowerError {
         )
     }
 
-    pub fn unused_warning(
-        input: Input,
-        errno: usize,
-        loc: Location,
-        name: &str,
-        caused_by: String,
-    ) -> Self {
-        let name = StyledString::new(readable_name(name), Some(WARN), Some(ATTR));
-        Self::new(
-            ErrorCore::new(
-                vec![SubMessage::only_loc(loc)],
-                switch_lang!(
-                    "japanese" => format!("{name}は使用されていません"),
-                    "simplified_chinese" => format!("{name}未使用"),
-                    "traditional_chinese" => format!("{name}未使用"),
-                    "english" => format!("{name} is not used"),
-                ),
-                errno,
-                UnusedWarning,
-                loc,
-            ),
-            input,
-            caused_by,
-        )
-    }
-
-    pub fn union_return_type_warning(
-        input: Input,
-        errno: usize,
-        loc: Location,
-        caused_by: String,
-        fn_name: &str,
-        typ: &Type,
-    ) -> Self {
-        let hint = switch_lang!(
-            "japanese" => format!("`{fn_name}(...): {typ} = ...`など明示的に戻り値型を指定してください"),
-            "simplified_chinese" => format!("请明确指定函数{fn_name}的返回类型，例如`{fn_name}(...): {typ} = ...`"),
-            "traditional_chinese" => format!("請明確指定函數{fn_name}的返回類型，例如`{fn_name}(...): {typ} = ...`"),
-            "english" => format!("please explicitly specify the return type of function {fn_name}, for example `{fn_name}(...): {typ} = ...`"),
-        );
-        LowerError::new(
-            ErrorCore::new(
-                vec![SubMessage::ambiguous_new(loc, vec![], Some(hint))],
-                switch_lang!(
-                    "japanese" => format!("関数{fn_name}の戻り値型が単一ではありません"),
-                    "simplified_chinese" => format!("函数{fn_name}的返回类型不是单一的"),
-                    "traditional_chinese" => format!("函數{fn_name}的返回類型不是單一的"),
-                    "english" => format!("the return type of function {fn_name} is not single"),
-                ),
-                errno,
-                TypeWarning,
-                loc,
-            ),
-            input,
-            caused_by,
-        )
-    }
-
     pub fn del_error(input: Input, errno: usize, ident: &Identifier, caused_by: String) -> Self {
         let name = StyledString::new(readable_name(ident.inspect()), Some(WARN), Some(ATTR));
         Self::new(
@@ -2476,6 +2418,92 @@ impl LowerError {
     }
 }
 
+impl LowerWarning {
+    pub fn unused_warning(
+        input: Input,
+        errno: usize,
+        loc: Location,
+        name: &str,
+        caused_by: String,
+    ) -> Self {
+        let name = StyledString::new(readable_name(name), Some(WARN), Some(ATTR));
+        Self::new(
+            ErrorCore::new(
+                vec![SubMessage::only_loc(loc)],
+                switch_lang!(
+                    "japanese" => format!("{name}は使用されていません"),
+                    "simplified_chinese" => format!("{name}未使用"),
+                    "traditional_chinese" => format!("{name}未使用"),
+                    "english" => format!("{name} is not used"),
+                ),
+                errno,
+                UnusedWarning,
+                loc,
+            ),
+            input,
+            caused_by,
+        )
+    }
+
+    pub fn union_return_type_warning(
+        input: Input,
+        errno: usize,
+        loc: Location,
+        caused_by: String,
+        fn_name: &str,
+        typ: &Type,
+    ) -> Self {
+        let hint = switch_lang!(
+            "japanese" => format!("`{fn_name}(...): {typ} = ...`など明示的に戻り値型を指定してください"),
+            "simplified_chinese" => format!("请明确指定函数{fn_name}的返回类型，例如`{fn_name}(...): {typ} = ...`"),
+            "traditional_chinese" => format!("請明確指定函數{fn_name}的返回類型，例如`{fn_name}(...): {typ} = ...`"),
+            "english" => format!("please explicitly specify the return type of function {fn_name}, for example `{fn_name}(...): {typ} = ...`"),
+        );
+        LowerError::new(
+            ErrorCore::new(
+                vec![SubMessage::ambiguous_new(loc, vec![], Some(hint))],
+                switch_lang!(
+                    "japanese" => format!("関数{fn_name}の戻り値型が単一ではありません"),
+                    "simplified_chinese" => format!("函数{fn_name}的返回类型不是单一的"),
+                    "traditional_chinese" => format!("函數{fn_name}的返回類型不是單一的"),
+                    "english" => format!("the return type of function {fn_name} is not single"),
+                ),
+                errno,
+                TypeWarning,
+                loc,
+            ),
+            input,
+            caused_by,
+        )
+    }
+
+    pub fn builtin_exists_warning(
+        input: Input,
+        errno: usize,
+        loc: Location,
+        caused_by: String,
+        name: &str,
+    ) -> Self {
+        let name = StyledStr::new(readable_name(name), Some(WARN), Some(ATTR));
+        Self::new(
+            ErrorCore::new(
+                vec![SubMessage::only_loc(loc)],
+                switch_lang!(
+                    "japanese" => format!("同名の組み込み関数{name}が既に存在します"),
+                    "simplified_chinese" => format!("已存在同名的内置函数{name}"),
+                    "traditional_chinese" => format!("已存在同名的內置函數{name}"),
+                    "english" => format!("a built-in function named {name} already exists"),
+                ),
+                errno,
+                NameWarning,
+                loc,
+            ),
+            input,
+            caused_by,
+        )
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct CompileErrors(Vec<CompileError>);
 
diff --git a/compiler/erg_compiler/lower.rs b/compiler/erg_compiler/lower.rs
index b03efcd5..17fa2259 100644
--- a/compiler/erg_compiler/lower.rs
+++ b/compiler/erg_compiler/lower.rs
@@ -1059,6 +1059,19 @@ impl ASTLowerer {
                 self.ctx.caused_by(),
                 &name,
             )));
+        } else if self
+            .ctx
+            .get_builtins()
+            .and_then(|ctx| ctx.get_var_info(&name))
+            .is_some()
+        {
+            self.warns.push(LowerWarning::builtin_exists_warning(
+                self.cfg.input.clone(),
+                line!() as usize,
+                def.sig.loc(),
+                self.ctx.caused_by(),
+                &name,
+            ));
         }
         let kind = ContextKind::from(def.def_kind());
         let vis = def.sig.vis();