fix: refinement type assert cast bug

Author: mtshiba

crates/erg_common/triple.rs

@@ -141,6 +141,14 @@ impl<T> Triple<T, T> {
             Triple::Ok(a) | Triple::Err(a) => Some(a),
         }
     }
+
+    pub fn merge_or(self, default: T) -> T {
+        match self {
+            Triple::None => default,
+            Triple::Ok(ok) => ok,
+            Triple::Err(err) => err,
+        }
+    }
 }
 
 impl<T, E: std::error::Error> Triple<T, E> {

crates/erg_compiler/context/inquire.rs

@@ -3657,6 +3657,7 @@ impl Context {
     /// ```erg
     /// recover_typarams(Int, Nat) == Nat
     /// recover_typarams(Array!(Int, _), Array(Nat, 2)) == Array!(Nat, 2)
+    /// recover_typarams(Str or NoneType, {"a", "b"}) == {"a", "b"}
     /// ```
     /// ```erg
     /// # REVIEW: should be?
@@ -3667,7 +3668,8 @@ impl Context {
         let is_never =
             self.subtype_of(&intersec, &Type::Never) && guard.to.as_ref() != &Type::Never;
         if !is_never {
-            return Ok(intersec);
+            let min = self.min(&intersec, &guard.to).merge_or(&intersec);
+            return Ok(min.clone());
         }
         if guard.to.is_monomorphic() {
             if self.related(base, &guard.to) {

crates/erg_compiler/tests/infer.er

@@ -30,3 +30,11 @@ c_new x, y = C.new x, y
 C = Class Int
 C.
     new x, y = Self x + y
+
+val!() =
+    for! [{ "a": "b" }], (pkg as {Str: Str}) =>
+        x = pkg.get("a", "c")
+        assert x in {"b"}
+        val!::return x
+    "d"
+val = val!()

crates/erg_compiler/tests/test.rs

@@ -87,6 +87,9 @@ fn _test_infer_types() -> Result<(), ()> {
     let c_new_t = func2(add_r, r, c.clone()).quantify();
     module.context.assert_var_type("c_new", &c_new_t)?;
     module.context.assert_attr_type(&c, "new", &c_new_t)?;
+    module
+        .context
+        .assert_var_type("val", &v_enum(set! { "b".into(), "d".into() }))?;
     Ok(())
 }