diff --git a/crates/erg_common/shared.rs b/crates/erg_common/shared.rs
index d0b4e9e2d..215271b10 100644
--- a/crates/erg_common/shared.rs
+++ b/crates/erg_common/shared.rs
@@ -144,6 +144,8 @@ impl<T: ?Sized> Shared<T> {
     #[track_caller]
     pub fn borrow(&self) -> RwLockReadGuard<'_, T> {
         self.wait_until_unlocked();
+        #[cfg(any(feature = "backtrace", feature = "debug"))]
+        let first_catcher = self.data.try_write_for(GET_TIMEOUT).is_some();
         let res = self.data.try_read_for(GET_TIMEOUT).unwrap_or_else(|| {
             #[cfg(any(feature = "backtrace", feature = "debug"))]
             {
@@ -159,7 +161,7 @@ impl<T: ?Sized> Shared<T> {
             }
         });
         #[cfg(any(feature = "backtrace", feature = "debug"))]
-        {
+        if first_catcher {
             *self.last_borrowed_at.try_write_for(GET_TIMEOUT).unwrap() =
                 BorrowInfo::new(Some(std::panic::Location::caller()));
         }
@@ -187,8 +189,6 @@ impl<T: ?Sized> Shared<T> {
         #[cfg(any(feature = "backtrace", feature = "debug"))]
         {
             let caller = std::panic::Location::caller();
-            *self.last_borrowed_at.try_write_for(SET_TIMEOUT).unwrap() =
-                BorrowInfo::new(Some(caller));
             *self
                 .last_mut_borrowed_at
                 .try_write_for(SET_TIMEOUT)
diff --git a/crates/erg_compiler/context/compare.rs b/crates/erg_compiler/context/compare.rs
index dea8f8c4d..8e3dc2908 100644
--- a/crates/erg_compiler/context/compare.rs
+++ b/crates/erg_compiler/context/compare.rs
@@ -7,7 +7,7 @@ use erg_common::dict::Dict;
 use erg_common::set::Set;
 use erg_common::style::colors::DEBUG_ERROR;
 use erg_common::traits::StructuralEq;
-use erg_common::{assume_unreachable, log};
+use erg_common::{assume_unreachable, log, set_recursion_limit};
 use erg_common::{Str, Triple};
 
 use crate::context::eval::UndoableLinkedList;
@@ -355,6 +355,7 @@ impl Context {
     /// 単一化、評価等はここでは行わない、スーパータイプになる **可能性があるか** だけ判定する
     /// ので、lhsが(未連携)型変数の場合は単一化せずにtrueを返す
     pub(crate) fn structural_supertype_of(&self, lhs: &Type, rhs: &Type) -> bool {
+        set_recursion_limit!(false, 128);
         match (lhs, rhs) {
             // Proc :> Func if params are compatible
             // * default params can be omitted (e.g. (Int, x := Int) -> Int <: (Int) -> Int)
diff --git a/crates/erg_compiler/context/inquire.rs b/crates/erg_compiler/context/inquire.rs
index a36dfc7ff..70ea0c5be 100644
--- a/crates/erg_compiler/context/inquire.rs
+++ b/crates/erg_compiler/context/inquire.rs
@@ -1294,9 +1294,11 @@ impl Context {
                 })
                 .collect::<Vec<_>>();
             let return_t = free_var(self.level, Constraint::new_type_of(Type));
-            let subr_t = fn_met(obj.t(), nd_params, None, d_params, None, return_t);
+            let mut subr_t = fn_met(obj.t(), nd_params, None, d_params, None, return_t);
             if let Some(fv) = obj.ref_t().as_free() {
                 if let Some((_sub, sup)) = fv.get_subsup() {
+                    // avoid recursion
+                    *subr_t.mut_self_t().unwrap() = Never;
                     let vis = self
                         .instantiate_vis_modifier(&attr_name.vis)
                         .unwrap_or(VisibilityModifier::Public);
diff --git a/crates/erg_compiler/ty/free.rs b/crates/erg_compiler/ty/free.rs
index 36edcd83f..7a68a03dc 100644
--- a/crates/erg_compiler/ty/free.rs
+++ b/crates/erg_compiler/ty/free.rs
@@ -739,6 +739,9 @@ impl<T: Send + Clone> Free<T> {
     pub fn forced_as_ref(&self) -> &FreeKind<T> {
         unsafe { self.as_ptr().as_ref() }.unwrap()
     }
+    pub fn forced_as_mut(&mut self) -> &mut FreeKind<T> {
+        unsafe { self.as_ptr().as_mut() }.unwrap()
+    }
 }
 
 impl Free<Type> {
diff --git a/crates/erg_compiler/ty/mod.rs b/crates/erg_compiler/ty/mod.rs
index 5c798dd79..f11b66679 100644
--- a/crates/erg_compiler/ty/mod.rs
+++ b/crates/erg_compiler/ty/mod.rs
@@ -3886,6 +3886,18 @@ impl Type {
         }
     }
 
+    pub fn mut_self_t(&mut self) -> Option<&mut Type> {
+        match self {
+            Self::FreeVar(fv) if fv.is_linked() => {
+                fv.forced_as_mut().linked_mut().and_then(|t| t.mut_self_t())
+            }
+            Self::Refinement(refine) => refine.t.mut_self_t(),
+            Self::Subr(subr) => subr.mut_self_t(),
+            Self::Quantified(quant) => quant.mut_self_t(),
+            _ => None,
+        }
+    }
+
     pub fn non_default_params(&self) -> Option<&Vec<ParamTy>> {
         match self {
             Self::FreeVar(fv) if fv.is_linked() => fv
@@ -3999,6 +4011,10 @@ impl Type {
 
     pub fn mut_return_t(&mut self) -> Option<&mut Type> {
         match self {
+            Self::FreeVar(fv) if fv.is_linked() => fv
+                .forced_as_mut()
+                .linked_mut()
+                .and_then(|t| t.mut_return_t()),
             Self::Refinement(refine) => refine.t.mut_return_t(),
             Self::Subr(SubrType { return_t, .. }) | Self::Callable { return_t, .. } => {
                 Some(return_t)