fix: #415

crates/erg_compiler/hir.rs

@@ -1377,6 +1377,9 @@ impl HasType for Call {
         if let Some(attr) = self.attr_name.as_mut() {
             Some(attr.ref_mut_t())
         } else {
+            if let Expr::Call(call) = self.obj.as_ref() {
+                call.return_t()?;
+            }
             Some(self.obj.ref_mut_t())
         }
     }
@@ -1417,6 +1420,14 @@ impl Call {
             }
         })
     }
+
+    pub fn return_t(&self) -> Option<&Type> {
+        if let Some(attr) = self.attr_name.as_ref() {
+            attr.ref_t().return_t()
+        } else {
+            self.obj.ref_t().return_t()
+        }
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]

crates/erg_compiler/lower.rs

@@ -1024,13 +1024,21 @@ impl ASTLowerer {
                 .module
                 .context
                 .subtype_of(vi.t.return_t().unwrap(), &Type::Bool));
-            *vi.t.mut_return_t().unwrap() = guard;
+            if let Some(ret_t) = vi.t.mut_return_t() {
+                *ret_t = guard;
+            }
         }
         let attr_name = if let Some(attr_name) = call.attr_name {
             self.inc_ref(&vi, &attr_name.name);
             Some(hir::Identifier::new(attr_name, None, vi))
         } else {
-            *obj.ref_mut_t() = vi.t;
+            if let hir::Expr::Call(call) = &obj {
+                if call.return_t().is_some() {
+                    *obj.ref_mut_t() = vi.t;
+                }
+            } else {
+                *obj.ref_mut_t() = vi.t;
+            }
             None
         };
         let mut call = hir::Call::new(obj, attr_name, hir_args);

crates/erg_compiler/ty/mod.rs

@@ -45,6 +45,7 @@ pub trait HasType {
     // 関数呼び出しの場合、.ref_t()は戻り値を返し、signature_t()は関数全体の型を返す
     fn signature_t(&self) -> Option<&Type>;
     // 最後にHIR全体の型変数を消すために使う
+    /// `x.ref_mut_t()` may panic, in which case `x` is `Call` and `x.ref_t() == Type::Failure`.
     fn ref_mut_t(&mut self) -> &mut Type;
     fn signature_mut_t(&mut self) -> Option<&mut Type>;
     #[inline]