NNBD: Dart throws error when method with nullable required parameter overrides legacy one

[iarkh]

Dart VM version: 2.8.0-dev.20.0 (dev) (Fri Apr 3 10:19:55 2020 +0200) on "windows_x64"

In the following source code example method with nullable required parameter override legacy one:

test_out_lib.dart:

// @dart=2.6
class LEGACY_REQUIRED_ARGS {
  void test_default   ({int i = 1}) {}
  void test_nondefault({int i    }) {}
}

test.dart:

import "test_out_lib.dart";

class A extends LEGACY_REQUIRED_ARGS {
  void test_default   ({required int? i}) { print(i); }
  void test_nondefault({required int? i}) { print(i); }
}

main() {
  A().test_default(i: 1);
  A().test_nondefault(i: 1);
}

It passes with analyzer and throws a compile error with dart:

$> dart --enable-experiment=non-nullable test.dart
test.dart:4:39: Error: The required named parameter 'i' in method 'A.test_default' is not required in overridden method 'LEGACY_REQUIRED_ARGS.test_default'.
void test_default ({required int? i}) { print(i); }
^
test_out_lib.dart:3:8: Context: This is the overridden method ('test_default').
void test_default ({int i = 1}) {}
^
test.dart:5:39: Error: The required named parameter 'i' in method 'A.test_nondefault' is not required in overridden method 'LEGACY_REQUIRED_ARGS.test_nondefault'.
void test_nondefault({required int? i}) { print(i); }
^
test_out_lib.dart:4:8: Context: This is the overridden method ('test_nondefault').
void test_nondefault({int i }) {}
^
$> dartanalyzer --enable-experiment=non-nullable test.dart
Analyzing test.dart...
No issues found!

Seems like dart should not throw error here too.
At least, both tools should behave similarly.

[eernstg]

It was concluded in dart-lang/language#812 that subtyping is adjusted such that a required named parameter can match a legacy named parameter. For example, void Function({required int i}) <: void* Function({int* i}). So in this case the CFE should accept the program.

A separate issue is the use of default values, cf. dart-lang/language#928.