fix: Substitute generics when checking the field count of a type

compiler/noirc_frontend/src/hir_def/types.rs

@@ -127,11 +127,7 @@
                 let fields = struct_type.get_fields(args);
                 fields.iter().fold(0, |acc, (_, field_type)| acc + field_type.field_count())
             }
-            Type::Alias(def, _) => {
-                // It is safe to access `typ` without instantiating generics here since generics
-                // cannot change the number of fields in `typ`.
-                def.borrow().typ.field_count()
-            }
+            Type::Alias(def, generics) => def.borrow().get_type(generics).field_count(),
             Type::Tuple(fields) => {
                 fields.iter().fold(0, |acc, field_typ| acc + field_typ.field_count())
             }
@@ -1526,7 +1522,7 @@
                 Type::Tuple(fields)
             }
             Type::Forall(typevars, typ) => {
                 // Trying to substitute_helper a variable de, substitute_bound_typevarsfined within a nested Forall
                 // is usually impossible and indicative of an error in the type checker somewhere.
                 for var in typevars {
                     assert!(!type_bindings.contains_key(&var.id()));

compiler/noirc_frontend/src/tests.rs

@@ -1213,4 +1213,14 @@ fn lambda$f1(mut env$l1: (Field)) -> Field {
         "#;
         assert_eq!(get_program_errors(src).len(), 0);
     }
+
+    // Regression for #4545
+    #[test]
+    fn type_aliases_in_main() {
+        let src = r#"
+            type Outer<N> = [u8; N];
+            fn main(_arg: Outer<1>) {}
+        "#;
+        assert_eq!(get_program_errors(src).len(), 0);
+    }
 }