fix: overload subtyping

crates/erg_compiler/context/compare.rs

@@ -1255,6 +1255,8 @@ impl Context {
             },
             (_, Not(r)) => self.diff(lhs, r),
             (Not(l), _) => self.diff(rhs, l),
+            // overloading
+            (l, r) if l.is_subr() && r.is_subr() => and(lhs.clone(), rhs.clone()),
             _ => self.simple_intersection(lhs, rhs),
         }
     }

crates/erg_compiler/declare.rs

@@ -49,7 +49,18 @@ impl ASTLowerer {
             _ => sig.inspect().cloned(),
         };
         let found_body_t = chunk.ref_t();
-        let ast::VarPattern::Ident(ident) = &sig.pat else { unreachable!() };
+        let ident = match &sig.pat {
+            ast::VarPattern::Ident(ident) => ident,
+            ast::VarPattern::Discard(token) => {
+                return Err(LowerErrors::from(LowerError::declare_error(
+                    self.cfg().input.clone(),
+                    line!() as usize,
+                    token.loc(),
+                    self.module.context.caused_by(),
+                )));
+            }
+            _ => unreachable!(),
+        };
         let id = body.id;
         if let Some(spec_t) = opt_spec_t {
             self.module

crates/erg_compiler/ty/mod.rs

@@ -1814,6 +1814,7 @@ impl Type {
             Self::Subr(_) => true,
             Self::Quantified(quant) => quant.is_subr(),
             Self::Refinement(refine) => refine.t.is_subr(),
+            Self::And(l, r) => l.is_subr() && r.is_subr(),
             _ => false,
         }
     }
@@ -1823,6 +1824,7 @@ impl Type {
             Self::FreeVar(fv) if fv.is_linked() => fv.crack().is_quantified_subr(),
             Self::Quantified(_) => true,
             Self::Refinement(refine) => refine.t.is_quantified_subr(),
+            Self::And(l, r) => l.is_quantified_subr() && r.is_quantified_subr(),
             _ => false,
         }
     }