Dart ignores non-nullability of variables and function parameters.

[iarkh]

Dart VM version: 2.7.0-dev.2.1 (Mon Dec 2 20:10:59 2019 +0100) on "windows_x64"

The following test example tries to assign null values to non-nullable variables (if nnbd is enabled):

class A {
  void test(int i) { print(i); }
}

main() {
  int i;
  i = null;
  print(i);
  A().test(1);
  A().test(null);
}

It throws a compile error with analyzer and prints null variable values with dart.

As i is non-nullable, it seems like dart should throw compile errors for the lines 7 and 10 too.

Sample output is:

$> dartanalyzer --enable-experiment=non-nullable test.dart
Analyzing test.dart...
error - A value of type 'Null' can't be assigned to a variable of type 'int'. - test.dart:7:7 - invalid_assignment
error - The argument type 'Null' can't be assigned to the parameter type 'int'. - test.dart:10:12 - argument_type_not_assignable
2 errors found.

$> dart --enable-experiment=non-nullable test.dart
null
1
null

[a-siva]

Thanks for your feedback, the non nullability language feature is still being implemented in the Dart VM and is not fully functional yet.

[chloestefantsova]

Quick status update on the CFE.

As of c85979a the CFE emits compile-time errors (or warnings, depending on the mode) for lines i = null; and A().test(null);. Here's how to test it:

Click to expand

$ cat /tmp/asdf.dart
class A {
  void test(int i) { print(i); }
}

main() {
  int i;
  i = null;
  print(i);
  A().test(1);
  A().test(null);
}

$ sdk/bin/dart pkg/front_end/tool/fasta.dart compile --dump-ir -o /dev/null --enable-experiment=non-nullable --force-nnbd-checks --nnbd-strong /tmp/asdf.dart
Running: .../sdk/bin/dart --enable-asserts .../pkg/front_end/tool/_fasta/compile.dart --dump-ir -o /dev/null --enable-experiment=non-nullable --force-nnbd-checks --nnbd-strong /tmp/asdf.dart
/tmp/asdf.dart:7:7: Error: A value of type 'Null?' can't be assigned to a variable of type 'int'.
  i = null;
      ^
/tmp/asdf.dart:10:12: Error: The argument type 'Null?' can't be assigned to the parameter type 'int'.
  A().test(null);
           ^
library;
//
// Problems in library:
//
// /tmp/asdf.dart:7:7: Error: A value of type 'Null?' can't be assigned to a variable of type 'int'.
//   i = null;
//       ^
//
// /tmp/asdf.dart:10:12: Error: The argument type 'Null?' can't be assigned to the parameter type 'int'.
//   A().test(null);
//            ^
//
import self as self;
import "dart:core" as core;

class A extends core::Object {
  synthetic constructor •() → self::A
    : super core::Object::•()
    ;
  method test(core::int i) → void {
    core::print(i);
  }
}
static method main() → dynamic {
  core::int i;
  i = let final<BottomType> #t1 = invalid-expression "/tmp/asdf.dart:7:7: Error: A value of type 'Null?' can't be assigned to a variable of type 'int'.\n  i = null;\n      ^" in null as{TypeError} core::int;
  core::print(i);
  new self::A::•().{self::A::test}(1);
  new self::A::•().{self::A::test}(let final<BottomType> #t2 = invalid-expression "/tmp/asdf.dart:10:12: Error: The argument type 'Null?' can't be assigned to the parameter type 'int'.\n  A().test(null);\n           ^" in null as{TypeError} core::int);
}

To compile in weak mode, remove flag --nnbd-strong:

Click to expand

$ sdk/bin/dart pkg/front_end/tool/fasta.dart compile --dump-ir -o /dev/null --enable-experiment=non-nullable --force-nnbd-checks /tmp/asdf.dart
Running: .../sdk/bin/dart --enable-asserts .../pkg/front_end/tool/_fasta/compile.dart --dump-ir -o /dev/null --enable-experiment=non-nullable --force-nnbd-checks /tmp/asdf.dart
/tmp/asdf.dart:7:3: Warning: Assigning value of type 'Null?' to a variable of type 'int'.
  i = null;
  ^
/tmp/asdf.dart:10:12: Warning: Assigning value of type 'Null?' to a variable of type 'int'.
  A().test(null);
           ^
library;
//
// Problems in library:
//
// /tmp/asdf.dart:7:3: Warning: Assigning value of type 'Null?' to a variable of type 'int'.
//   i = null;
//   ^
//
// /tmp/asdf.dart:10:12: Warning: Assigning value of type 'Null?' to a variable of type 'int'.
//   A().test(null);
//            ^
//
import self as self;
import "dart:core" as core;

class A extends core::Object {
  synthetic constructor •() → self::A
    : super core::Object::•()
    ;
  method test(core::int i) → void {
    core::print(i);
  }
}
static method main() → dynamic {
  core::int i;
  i = null;
  core::print(i);
  new self::A::•().{self::A::test}(1);
  new self::A::•().{self::A::test}(null);
}

[johnniwinther]

Removing the front-end label as this should be considered a vm issue now.

[a-siva]

@stefantsov Is the flag --force-nnbd-checks required to get these errors, because I checked and the VM does pass the --enable-experiment=non-nullable flag to the front end and it does not seem to report any errors.