diff --git a/compiler/erg_compiler/error.rs b/compiler/erg_compiler/error.rs
index ebf9f4db..81de17bb 100644
--- a/compiler/erg_compiler/error.rs
+++ b/compiler/erg_compiler/error.rs
@@ -2017,6 +2017,38 @@ impl LowerError {
         )
     }
 
+    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(
diff --git a/compiler/erg_compiler/lower.rs b/compiler/erg_compiler/lower.rs
index ab42b67a..c364af67 100644
--- a/compiler/erg_compiler/lower.rs
+++ b/compiler/erg_compiler/lower.rs
@@ -1170,6 +1170,18 @@ impl ASTLowerer {
                             body.id,
                             found_body_t,
                         )?;
+                        let return_t = t.return_t().unwrap();
+                        if return_t.union_types().is_some() && sig.return_t_spec.is_none() {
+                            let warn = LowerWarning::union_return_type_warning(
+                                self.input().clone(),
+                                line!() as usize,
+                                sig.loc(),
+                                self.ctx.caused_by(),
+                                sig.ident.inspect(),
+                                &Context::readable_type(return_t),
+                            );
+                            self.warns.push(warn);
+                        }
                         let mut ident = hir::Identifier::bare(sig.ident.dot, sig.ident.name);
                         ident.vi.t = t;
                         let sig = hir::SubrSignature::new(ident, params);
diff --git a/compiler/erg_compiler/ty/mod.rs b/compiler/erg_compiler/ty/mod.rs
index 4759c070..aabd272b 100644
--- a/compiler/erg_compiler/ty/mod.rs
+++ b/compiler/erg_compiler/ty/mod.rs
@@ -2203,7 +2203,7 @@ impl Type {
             // At least in situations where this function is needed, self cannot be Quantified.
             Self::Quantified(quant) => {
                 if quant.return_t().unwrap().is_generalized() {
-                    todo!("quantified return type (recursive function type inference)")
+                    log!(err "quantified return type (recursive function type inference)");
                 }
                 quant.return_t()
             }