fix: #510

crates/erg_compiler/link_hir.rs

@@ -233,6 +233,11 @@ impl<'a> HIRLinker<'a> {
                 match acc {
                     Accessor::Attr(attr) => {
                         self.replace_import(&mut attr.obj);
+                        if attr.ident.inspect() == "__file__"
+                            && attr.ident.vi.def_loc.module.is_none()
+                        {
+                            *expr = self.__file__();
+                        }
                     }
                     Accessor::Ident(ident) => match &ident.inspect()[..] {
                         "module" => {
@@ -241,6 +246,9 @@ impl<'a> HIRLinker<'a> {
                         "global" => {
                             *expr = Expr::from(Identifier::static_public("__builtins__"));
                         }
+                        "__file__" if ident.vi.def_loc.module.is_none() => {
+                            *expr = self.__file__();
+                        }
                         _ => {}
                     },
                 }
@@ -363,6 +371,22 @@ impl<'a> HIRLinker<'a> {
         Expr::from(__import__).call1(Expr::from(__name__))
     }
 
+    fn __file__(&self) -> Expr {
+        let path = self.cfg.input.path().to_path_buf();
+        let token = Token::new_fake(
+            TokenKind::StrLit,
+            format!(
+                "\"{}\"",
+                path.canonicalize().unwrap_or(path).to_string_lossy()
+            ),
+            0,
+            0,
+            0,
+        );
+        let lit = Literal::try_from(token).unwrap();
+        Expr::from(lit)
+    }
+
     /// ```erg
     /// x = import "mod"
     /// ```

tests/should_ok/dunder.er

@@ -0,0 +1,15 @@
+imp = import "import"
+
+func() =
+    __file__ = "foo"
+    __file__
+
+C = Class()
+C.
+    __file__ = "bar"
+
+assert __file__.endswith "dunder.er"
+assert func() == "foo"
+assert module::__file__.endswith "dunder.er"
+assert C.new().__file__ == "bar"
+assert imp.func().endswith "import.er"

tests/should_ok/import.er

@@ -0,0 +1 @@
+.func() = __file__

tests/test.rs

@@ -131,6 +131,11 @@ fn exec_dict_test() -> Result<(), ()> {
     expect_success("tests/should_ok/dict.er", 0)
 }
 
+#[test]
+fn exec_dunder() -> Result<(), ()> {
+    expect_success("tests/should_ok/dunder.er", 0)
+}
+
 #[test]
 fn exec_empty_check() -> Result<(), ()> {
     expect_success("tests/should_ok/dyn_type_check.er", 0)