fix: error msg for illegal use of Self type

crates/erg_compiler/context/instantiate_spec.rs

@@ -518,10 +518,11 @@ impl Context {
             "TraitType" => Ok(Type::TraitType),
             "Type" => Ok(Type::Type),
             "Self" => self.rec_get_self_t().ok_or_else(|| {
-                TyCheckErrors::from(TyCheckError::unreachable(
+                TyCheckErrors::from(TyCheckError::self_type_error(
                     self.cfg.input.clone(),
-                    erg_common::fn_name_full!(),
-                    line!(),
+                    line!() as usize,
+                    ident.loc(),
+                    self.caused_by(),
                 ))
             }),
             "True" | "False" | "None" => Err(TyCheckErrors::from(TyCheckError::not_a_type_error(

crates/erg_compiler/error/tycheck.rs

@@ -1326,4 +1326,23 @@ passed keyword args:    {kw_args_len}"
             caused_by,
         )
     }
+
+    pub fn self_type_error(input: Input, errno: usize, loc: Location, caused_by: String) -> Self {
+        Self::new(
+            ErrorCore::new(
+                vec![SubMessage::only_loc(loc)],
+                switch_lang!(
+                    "japanese" => format!("`Self`型はこの場所では使えません"),
+                    "simplified_chinese" => format!("`Self`类型不能用于此处"),
+                    "traditional_chinese" => format!("`Self`類型不能用於此處"),
+                    "english" => format!("`Self` type cannot be used here"),
+                ),
+                errno,
+                TypeError,
+                loc,
+            ),
+            input,
+            caused_by,
+        )
+    }
 }