diff --git a/compiler/erg_compiler/builder.rs b/compiler/erg_compiler/builder.rs
index 632bc593..bc9f5946 100644
--- a/compiler/erg_compiler/builder.rs
+++ b/compiler/erg_compiler/builder.rs
@@ -1,4 +1,4 @@
-use erg_common::config::{ErgConfig, Input};
+use erg_common::config::ErgConfig;
 use erg_common::traits::{Runnable, Stream};
 
 use erg_parser::ast::VarName;
@@ -31,25 +31,9 @@ impl HIRBuilder {
         }
     }
 
-    pub fn build_and_cache(&mut self, var_name: VarName) -> Result<(), CompileErrors> {
+    pub fn build_and_cache(&mut self, var_name: VarName, mode: &str) -> Result<(), CompileErrors> {
         let mut ast_builder = ASTBuilder::new(self.checker.cfg().copy());
         let ast = ast_builder.build()?;
-        let ast = Reorderer::new()
-            .reorder(ast)
-            .map_err(|errs| self.convert(errs))?;
-        let (hir, ctx) = self
-            .checker
-            .check(ast, "exec")
-            .map_err(|errs| self.convert(errs))?;
-        self.mod_cache.register(var_name, Some(hir), ctx);
-        Ok(())
-    }
-
-    pub fn build_and_cache_main(&mut self, src: String, mode: &str) -> Result<(), CompileErrors> {
-        let mut cfg = self.checker.cfg().copy();
-        cfg.input = Input::Str(src);
-        let mut ast_builder = ASTBuilder::new(cfg);
-        let ast = ast_builder.build()?;
         let ast = Reorderer::new()
             .reorder(ast)
             .map_err(|errs| self.convert(errs))?;
@@ -57,8 +41,7 @@ impl HIRBuilder {
             .checker
             .check(ast, mode)
             .map_err(|errs| self.convert(errs))?;
-        let name = VarName::from_static("<module>");
-        self.mod_cache.register(name, Some(hir), ctx);
+        self.mod_cache.register(var_name, Some(hir), ctx);
         Ok(())
     }
 }
diff --git a/compiler/erg_compiler/check.rs b/compiler/erg_compiler/check.rs
index 89716bbc..7e6ff692 100644
--- a/compiler/erg_compiler/check.rs
+++ b/compiler/erg_compiler/check.rs
@@ -1,4 +1,4 @@
-use erg_common::config::ErgConfig;
+use erg_common::config::{ErgConfig, Input};
 use erg_common::error::MultiErrorDisplay;
 use erg_common::traits::Runnable;
 
@@ -48,8 +48,12 @@ impl Runnable for Checker {
     }
 
     fn eval(&mut self, src: String) -> Result<String, TyCheckErrors> {
-        let mut builder = ASTBuilder::new(self.cfg().copy());
-        let ast = builder.build_with_str(src)?;
+        let cfg = ErgConfig {
+            input: Input::Str(src),
+            ..self.cfg().copy()
+        };
+        let mut builder = ASTBuilder::new(cfg);
+        let ast = builder.build()?;
         let (hir, _) = self.check(ast, "eval")?;
         Ok(hir.to_string())
     }
diff --git a/compiler/erg_compiler/compile.rs b/compiler/erg_compiler/compile.rs
index 1e411c1b..b62d2904 100644
--- a/compiler/erg_compiler/compile.rs
+++ b/compiler/erg_compiler/compile.rs
@@ -3,9 +3,10 @@
 //! コンパイラーを定義する
 use std::path::Path;
 
-use erg_common::config::ErgConfig;
+use erg_common::config::{ErgConfig, Input};
 use erg_common::log;
 use erg_common::traits::{Runnable, Stream};
+use erg_parser::ast::VarName;
 use erg_type::codeobj::CodeObj;
 
 use crate::builder::HIRBuilder;
@@ -148,8 +149,12 @@ impl Compiler {
 
     pub fn compile(&mut self, src: String, mode: &str) -> Result<CodeObj, CompileErrors> {
         log!(info "the compiling process has started.");
-        let mut hir_builder = HIRBuilder::new(self.cfg.copy(), self.mod_cache.clone());
-        hir_builder.build_and_cache_main(src, mode)?;
+        let cfg = ErgConfig {
+            input: Input::Str(src),
+            ..self.cfg.copy()
+        };
+        let mut hir_builder = HIRBuilder::new(cfg, self.mod_cache.clone());
+        hir_builder.build_and_cache(VarName::from_static("<module>"), mode)?;
         let hir = Linker::link(self.mod_cache.clone());
         let codeobj = self.code_generator.emit(hir);
         log!(info "code object:\n{}", codeobj.code_info());
diff --git a/compiler/erg_compiler/context/initialize/mod.rs b/compiler/erg_compiler/context/initialize/mod.rs
index 67e40645..57f4566c 100644
--- a/compiler/erg_compiler/context/initialize/mod.rs
+++ b/compiler/erg_compiler/context/initialize/mod.rs
@@ -1770,7 +1770,7 @@ impl Context {
         // ord.register_impl("__ge__", op_t,         Const, Public);
     }
 
-    pub(crate) fn init_builtins() -> Self {
+    pub(crate) fn init_builtins(mod_cache: &SharedModuleCache) {
         // TODO: capacityを正確に把握する
         let mut ctx = Context::module("<builtins>".into(), None, 40);
         ctx.init_builtin_funcs();
@@ -1780,15 +1780,15 @@ impl Context {
         ctx.init_builtin_traits();
         ctx.init_builtin_classes();
         ctx.init_builtin_patches();
-        ctx
+        mod_cache.register(VarName::from_static("<builtins>"), None, ctx);
     }
 
-    pub fn new_main_module(mod_cache: SharedModuleCache) -> Self {
+    pub fn new_module(mod_cache: SharedModuleCache) -> Self {
         Context::new(
-            "<module>".into(),
+            "<module>".into(), // TODO:
             ContextKind::Module,
             vec![],
-            Some(Context::init_builtins()),
+            None,
             Some(mod_cache),
             Context::TOP_LEVEL,
         )
diff --git a/compiler/erg_compiler/context/inquire.rs b/compiler/erg_compiler/context/inquire.rs
index 94ae3b95..6b1cb3b2 100644
--- a/compiler/erg_compiler/context/inquire.rs
+++ b/compiler/erg_compiler/context/inquire.rs
@@ -206,7 +206,7 @@ impl Context {
             self.validate_visibility(ident, vi, namespace)?;
             Ok(vi.t())
         } else {
-            if let Some(parent) = self.outer.as_ref() {
+            if let Some(parent) = self.get_outer().or_else(|| self.get_builtins()) {
                 return parent.rec_get_var_t(ident, namespace);
             }
             Err(TyCheckError::no_var_error(
@@ -267,7 +267,7 @@ impl Context {
             }
         }
         // TODO: dependent type widening
-        if let Some(parent) = self.outer.as_ref() {
+        if let Some(parent) = self.get_outer().or_else(|| self.get_builtins()) {
             parent.rec_get_attr_t(obj, ident, namespace)
         } else {
             Err(TyCheckError::no_attr_error(
@@ -849,7 +849,7 @@ impl Context {
         if let Some(obj) = self.consts.get(name.inspect()) {
             Ok(obj.clone())
         } else {
-            if let Some(parent) = self.outer.as_ref() {
+            if let Some(parent) = self.get_outer().or_else(|| self.get_builtins()) {
                 return parent.get_const_local(name, namespace);
             }
             Err(TyCheckError::no_var_error(
@@ -883,7 +883,7 @@ impl Context {
             }
         }
         // TODO: dependent type widening
-        if let Some(parent) = self.outer.as_ref() {
+        if let Some(parent) = self.get_outer().or_else(|| self.get_builtins()) {
             parent._get_const_attr(obj, name, namespace)
         } else {
             Err(TyCheckError::no_attr_error(
@@ -912,7 +912,7 @@ impl Context {
             .inspect();
         let len = most_similar_name.len();
         if levenshtein(most_similar_name, name) >= len / 2 {
-            let outer = self.outer.as_ref()?;
+            let outer = self.get_outer().or_else(|| self.get_builtins())?;
             outer.get_similar_name(name)
         } else {
             Some(most_similar_name)
@@ -1209,7 +1209,7 @@ impl Context {
         } else {
             vec![]
         };
-        if let Some(outer) = &self.outer {
+        if let Some(outer) = self.get_outer().or_else(|| self.get_builtins()) {
             [current, outer.rec_get_trait_impls(name)].concat()
         } else {
             current
@@ -1219,7 +1219,7 @@ impl Context {
     pub(crate) fn _rec_get_patch(&self, name: &VarName) -> Option<&Context> {
         if let Some(patch) = self.patches.get(name) {
             Some(patch)
-        } else if let Some(outer) = &self.outer {
+        } else if let Some(outer) = self.get_outer().or_else(|| self.get_builtins()) {
             outer._rec_get_patch(name)
         } else {
             None
@@ -1243,7 +1243,7 @@ impl Context {
                 return Some(val);
             }
         }
-        if let Some(outer) = &self.outer {
+        if let Some(outer) = self.get_outer().or_else(|| self.get_builtins()) {
             outer.rec_get_const_obj(name)
         } else {
             None
@@ -1253,7 +1253,7 @@ impl Context {
     pub(crate) fn rec_get_const_param_defaults(&self, name: &str) -> Option<&Vec<ConstTemplate>> {
         if let Some(impls) = self.const_param_defaults.get(name) {
             Some(impls)
-        } else if let Some(outer) = &self.outer {
+        } else if let Some(outer) = self.get_outer().or_else(|| self.get_builtins()) {
             outer.rec_get_const_param_defaults(name)
         } else {
             None
@@ -1271,7 +1271,7 @@ impl Context {
             } else {
                 None
             }
-        } else if let Some(outer) = &self.outer {
+        } else if let Some(outer) = self.get_outer().or_else(|| self.get_builtins()) {
             outer.rec_get_self_t()
         } else {
             None
@@ -1281,7 +1281,7 @@ impl Context {
     fn rec_get_mono_type(&self, name: &str) -> Option<(&Type, &Context)> {
         if let Some((t, ctx)) = self.mono_types.get(name) {
             Some((t, ctx))
-        } else if let Some(outer) = &self.outer {
+        } else if let Some(outer) = self.get_outer().or_else(|| self.get_builtins()) {
             outer.rec_get_mono_type(name)
         } else {
             None
@@ -1291,7 +1291,7 @@ impl Context {
     fn rec_get_poly_type(&self, name: &str) -> Option<(&Type, &Context)> {
         if let Some((t, ctx)) = self.poly_types.get(name) {
             Some((t, ctx))
-        } else if let Some(outer) = &self.outer {
+        } else if let Some(outer) = self.get_outer().or_else(|| self.get_builtins()) {
             outer.rec_get_poly_type(name)
         } else {
             None
@@ -1302,6 +1302,7 @@ impl Context {
         if let Some((t, ctx)) = self.mono_types.get_mut(name) {
             Some((t, ctx))
         } else if let Some(outer) = self.outer.as_mut() {
+            // builtins cannot be got as mutable
             outer.rec_get_mut_mono_type(name)
         } else {
             None
@@ -1323,7 +1324,7 @@ impl Context {
             Some((t, ctx))
         } else if let Some((t, ctx)) = self.poly_types.get(name) {
             Some((t, ctx))
-        } else if let Some(outer) = &self.outer {
+        } else if let Some(outer) = self.get_outer().or_else(|| self.get_builtins()) {
             outer.rec_get_type(name)
         } else {
             None
diff --git a/compiler/erg_compiler/context/mod.rs b/compiler/erg_compiler/context/mod.rs
index 1ca3a86e..38abd27c 100644
--- a/compiler/erg_compiler/context/mod.rs
+++ b/compiler/erg_compiler/context/mod.rs
@@ -19,7 +19,6 @@ use std::option::Option; // conflicting to Type::Option
 
 use erg_common::astr::AtomicStr;
 use erg_common::dict::Dict;
-use erg_common::error::Location;
 use erg_common::impl_display_from_debug;
 use erg_common::traits::{Locational, Stream};
 use erg_common::vis::Visibility;
@@ -554,6 +553,23 @@ impl Context {
         AtomicStr::arc(&self.name[..])
     }
 
+    pub(crate) fn get_outer(&self) -> Option<&Context> {
+        self.outer.as_ref().map(|x| x.as_ref())
+    }
+
+    /// Returns None if self is `<builtins>`.
+    /// This avoids infinite loops.
+    pub(crate) fn get_builtins(&self) -> Option<&Context> {
+        // builtins中で定義した型等はmod_cacheがNoneになっている
+        if &self.name[..] != "<builtins>" {
+            self.mod_cache
+                .as_ref()
+                .map(|cache| cache.ref_ctx("<builtins>").unwrap())
+        } else {
+            None
+        }
+    }
+
     pub(crate) fn grow(
         &mut self,
         name: &str,
@@ -567,7 +583,7 @@ impl Context {
         };
         log!(info "{}: current namespace: {name}", fn_name!());
         self.outer = Some(Box::new(mem::take(self)));
-        self.mod_cache = self.outer.as_ref().unwrap().mod_cache.clone();
+        self.mod_cache = self.get_outer().unwrap().mod_cache.clone();
         self.name = name.into();
         self.kind = kind;
         Ok(())
@@ -584,7 +600,7 @@ impl Context {
                 &vi.t,
             ));
         }
-        if let Some(parent) = &mut self.outer {
+        if let Some(parent) = self.outer.as_mut() {
             let parent = mem::take(parent);
             let ctx = mem::take(self);
             *self = *parent;
@@ -595,12 +611,14 @@ impl Context {
                 Ok(ctx)
             }
         } else {
-            Err(TyCheckErrors::from(TyCheckError::checker_bug(
-                0,
-                Location::Unknown,
-                fn_name!(),
-                line!(),
-            )))
+            // toplevel
+            let ctx = mem::take(self);
+            log!(info "{}: current namespace: {}", fn_name!(), self.name);
+            if !uninited_errs.is_empty() {
+                Err(uninited_errs)
+            } else {
+                Ok(ctx)
+            }
         }
     }
 }
diff --git a/compiler/erg_compiler/context/register.rs b/compiler/erg_compiler/context/register.rs
index c4a0a83c..a35a6ea0 100644
--- a/compiler/erg_compiler/context/register.rs
+++ b/compiler/erg_compiler/context/register.rs
@@ -48,7 +48,7 @@ impl Context {
             return Some((name, vi));
         }
         if is_const {
-            if let Some(outer) = &self.outer {
+            if let Some(outer) = self.get_outer().or_else(|| self.get_builtins()) {
                 outer.registered_info(name, is_const)
             } else {
                 None
@@ -871,7 +871,7 @@ impl Context {
             ..ErgConfig::default()
         };
         let mut hir_builder = HIRBuilder::new(cfg, mod_cache.clone());
-        if let Err(errs) = hir_builder.build_and_cache(var_name.clone()) {
+        if let Err(errs) = hir_builder.build_and_cache(var_name.clone(), "exec") {
             errs.fmt_all_stderr();
         }
     }
diff --git a/compiler/erg_compiler/lower.rs b/compiler/erg_compiler/lower.rs
index 89e9c6df..52c65c26 100644
--- a/compiler/erg_compiler/lower.rs
+++ b/compiler/erg_compiler/lower.rs
@@ -3,7 +3,7 @@
 //! ASTLowerer(ASTからHIRへの変換器)を実装
 
 use erg_common::astr::AtomicStr;
-use erg_common::config::ErgConfig;
+use erg_common::config::{ErgConfig, Input};
 use erg_common::error::{Location, MultiErrorDisplay};
 use erg_common::set::Set;
 use erg_common::traits::{Locational, Runnable, Stream};
@@ -81,8 +81,12 @@ impl Runnable for ASTLowerer {
     }
 
     fn eval(&mut self, src: String) -> Result<String, CompileErrors> {
-        let mut ast_builder = ASTBuilder::new(self.cfg.copy());
-        let ast = ast_builder.build_with_str(src)?;
+        let cfg = ErgConfig {
+            input: Input::Str(src),
+            ..self.cfg.copy()
+        };
+        let mut ast_builder = ASTBuilder::new(cfg);
+        let ast = ast_builder.build()?;
         let (hir, ..) = self.lower(ast, "eval").map_err(|errs| self.convert(errs))?;
         Ok(format!("{hir}"))
     }
@@ -92,7 +96,7 @@ impl ASTLowerer {
     pub fn new_with_cache(cfg: ErgConfig, mod_cache: SharedModuleCache) -> Self {
         Self {
             cfg,
-            ctx: Context::new_main_module(mod_cache),
+            ctx: Context::new_module(mod_cache),
             errs: LowerErrors::empty(),
             warns: LowerWarnings::empty(),
         }
diff --git a/compiler/erg_compiler/mod_cache.rs b/compiler/erg_compiler/mod_cache.rs
index 8c508277..57aedd28 100644
--- a/compiler/erg_compiler/mod_cache.rs
+++ b/compiler/erg_compiler/mod_cache.rs
@@ -128,7 +128,9 @@ impl fmt::Display for SharedModuleCache {
 
 impl SharedModuleCache {
     pub fn new() -> Self {
-        Self(Shared::new(ModuleCache::new()))
+        let self_ = Self(Shared::new(ModuleCache::new()));
+        Context::init_builtins(&self_);
+        self_
     }
 
     pub fn get_ctx<Q: Eq + Hash + ?Sized>(&self, name: &Q) -> Option<Rc<Context>>
diff --git a/compiler/erg_compiler/tests/test.rs b/compiler/erg_compiler/tests/test.rs
index fa286655..d3539589 100644
--- a/compiler/erg_compiler/tests/test.rs
+++ b/compiler/erg_compiler/tests/test.rs
@@ -3,14 +3,14 @@ use erg_compiler::mod_cache::SharedModuleCache;
 
 #[test]
 fn test_subtyping() -> Result<(), ()> {
-    let context = Context::new_main_module(SharedModuleCache::new());
+    let context = Context::new_module(SharedModuleCache::new());
     context.test_refinement_subtyping()?;
     Ok(())
 }
 
 #[test]
 fn test_instantiation_and_generalization() -> Result<(), ()> {
-    let context = Context::new_main_module(SharedModuleCache::new());
+    let context = Context::new_module(SharedModuleCache::new());
     context.test_instantiation_and_generalization()?;
     Ok(())
 }
diff --git a/compiler/erg_parser/builder.rs b/compiler/erg_parser/builder.rs
index bd7c24c9..68723b67 100644
--- a/compiler/erg_parser/builder.rs
+++ b/compiler/erg_parser/builder.rs
@@ -25,17 +25,7 @@ impl ASTBuilder {
         let module = desugarer.desugar(module);
         let mut desugarer = Desugarer::new();
         let module = desugarer.desugar(module);
-        let ast = AST::new(Str::ever(self.runner.cfg().module), module);
-        Ok(ast)
-    }
-
-    pub fn build_with_str(&mut self, src: String) -> Result<AST, ParserRunnerErrors> {
-        let module = self.runner.parse_with_str(src)?;
-        let mut desugarer = Desugarer::new();
-        let module = desugarer.desugar(module);
-        let mut desugarer = Desugarer::new();
-        let module = desugarer.desugar(module);
-        let ast = AST::new(Str::ever(self.runner.cfg().module), module);
+        let ast = AST::new(Str::rc(self.runner.cfg().input.filename()), module);
         Ok(ast)
     }
 }
diff --git a/compiler/erg_type/free.rs b/compiler/erg_type/free.rs
index d5477352..4dea787a 100644
--- a/compiler/erg_type/free.rs
+++ b/compiler/erg_type/free.rs
@@ -337,6 +337,20 @@ impl<T> FreeKind<T> {
             _ => None,
         }
     }
+
+    pub const fn linked(&self) -> Option<&T> {
+        match self {
+            Self::Linked(typ) => Some(typ),
+            _ => None,
+        }
+    }
+
+    pub fn linked_mut(&mut self) -> Option<&mut T> {
+        match self {
+            Self::Linked(typ) => Some(typ),
+            _ => None,
+        }
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
diff --git a/compiler/erg_type/lib.rs b/compiler/erg_type/lib.rs
index 2784a7df..453bcdd0 100644
--- a/compiler/erg_type/lib.rs
+++ b/compiler/erg_type/lib.rs
@@ -1104,6 +1104,10 @@ pub enum Type {
     Ellipsis, // これはクラスのほうで型推論用のマーカーではない
     Never,    // {}
     Mono(Str),
+    ForeignMono {
+        path: Str,
+        name: Str,
+    },
     /* Polymorphic types */
     Ref(Box<Type>),
     RefMut {
@@ -1174,6 +1178,10 @@ impl PartialEq for Type {
             | (Self::Ellipsis, Self::Ellipsis)
             | (Self::Never, Self::Never) => true,
             (Self::Mono(l), Self::Mono(r)) => l == r,
+            (
+                Self::ForeignMono { path: lp, name: ln },
+                Self::ForeignMono { path: rp, name: rn },
+            ) => lp == rp && ln == rn,
             (Self::MonoQVar(l), Self::MonoQVar(r)) => l == r,
             (Self::Ref(l), Self::Ref(r)) => l == r,
             (
@@ -1269,6 +1277,7 @@ impl LimitedDisplay for Type {
         }
         match self {
             Self::Mono(name) => write!(f, "{name}"),
+            Self::ForeignMono { path, name } => write!(f, "{name}(of {path})"),
             Self::Ref(t) => {
                 write!(f, "{}(", self.name())?;
                 t.limited_fmt(f, limit - 1)?;
@@ -1758,7 +1767,9 @@ impl Type {
             Self::Error => Str::ever("Error"),
             Self::Inf => Str::ever("Inf"),
             Self::NegInf => Str::ever("NegInf"),
-            Self::Mono(name) | Self::MonoQVar(name) => name.clone(),
+            Self::Mono(name) | Self::MonoQVar(name) | Self::ForeignMono { name, .. } => {
+                name.clone()
+            }
             Self::And(_, _) => Str::ever("And"),
             Self::Not(_, _) => Str::ever("Not"),
             Self::Or(_, _) => Str::ever("Or"),
@@ -1991,16 +2002,22 @@ impl Type {
 
     pub fn self_t(&self) -> Option<&Type> {
         match self {
-            Self::FreeVar(fv) if fv.is_linked() => todo!("linked: {fv}"),
+            Self::FreeVar(fv) if fv.is_linked() => unsafe { fv.as_ptr().as_ref() }
+                .unwrap()
+                .linked()
+                .and_then(|t| t.self_t()),
             Self::Refinement(refine) => refine.t.self_t(),
             Self::Subr(subr) => subr.self_t(),
             _ => None,
         }
     }
 
-    pub const fn non_default_params(&self) -> Option<&Vec<ParamTy>> {
+    pub fn non_default_params(&self) -> Option<&Vec<ParamTy>> {
         match self {
-            Self::FreeVar(_) => panic!("fv"),
+            Self::FreeVar(fv) if fv.is_linked() => unsafe { fv.as_ptr().as_ref() }
+                .unwrap()
+                .linked()
+                .and_then(|t| t.non_default_params()),
             Self::Refinement(refine) => refine.t.non_default_params(),
             Self::Subr(SubrType {
                 non_default_params, ..
@@ -2012,7 +2029,10 @@ impl Type {
 
     pub fn var_args(&self) -> Option<&ParamTy> {
         match self {
-            Self::FreeVar(_) => panic!("fv"),
+            Self::FreeVar(fv) if fv.is_linked() => unsafe { fv.as_ptr().as_ref() }
+                .unwrap()
+                .linked()
+                .and_then(|t| t.var_args()),
             Self::Refinement(refine) => refine.t.var_args(),
             Self::Subr(SubrType {
                 var_params: var_args,
@@ -2023,18 +2043,24 @@ impl Type {
         }
     }
 
-    pub const fn default_params(&self) -> Option<&Vec<ParamTy>> {
+    pub fn default_params(&self) -> Option<&Vec<ParamTy>> {
         match self {
-            Self::FreeVar(_) => panic!("fv"),
+            Self::FreeVar(fv) if fv.is_linked() => unsafe { fv.as_ptr().as_ref() }
+                .unwrap()
+                .linked()
+                .and_then(|t| t.default_params()),
             Self::Refinement(refine) => refine.t.default_params(),
             Self::Subr(SubrType { default_params, .. }) => Some(default_params),
             _ => None,
         }
     }
 
-    pub const fn return_t(&self) -> Option<&Type> {
+    pub fn return_t(&self) -> Option<&Type> {
         match self {
-            Self::FreeVar(_) => panic!("fv"),
+            Self::FreeVar(fv) if fv.is_linked() => unsafe { fv.as_ptr().as_ref() }
+                .unwrap()
+                .linked()
+                .and_then(|t| t.return_t()),
             Self::Refinement(refine) => refine.t.return_t(),
             Self::Subr(SubrType { return_t, .. }) | Self::Callable { return_t, .. } => {
                 Some(return_t)
@@ -2045,7 +2071,10 @@ impl Type {
 
     pub fn mut_return_t(&mut self) -> Option<&mut Type> {
         match self {
-            Self::FreeVar(_) => panic!("fv"),
+            Self::FreeVar(fv) if fv.is_linked() => unsafe { fv.as_ptr().as_mut() }
+                .unwrap()
+                .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)