NNBD: Analyzer throws a compile error when async function returns dynamic

[iarkh]

Dart VM version: 2.8.0-dev.18.0 (dev) (Fri Mar 27 11:10:52 2020 +0100) on "windows_x64"

The following sample code example fails with analyzer on the line 3 and passes with dart:

dynamic getNull() => null;

Future<bool> test1() async => await getNull();
Future<bool> test2() => getNull();
bool test3() => getNull();

main() {}

Seems like there should not be compile error here.

Please note that analyzer is OK with the lines 4 and 5.

Sample output is:

$> dartanalyzer --enable-experiment=non-nullable test.dart
Analyzing test.dart...
error - A value of type 'dynamic' can't be returned from function 'test1' because it has a return type of 'bool'. - test.dart:3:31 - return_of_invalid_type
1 error found.

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

[eernstg]

Right, something happened to the checks in the analyzer for a returned expression of type dynamic in an async (=> or {}) function around version 2.3 (where n018.dart contains the example given here):

> /usr/local/google/home/eernst/bin/dart-sdk-2.2.1-dev.3.0/bin/dartanalyzer --enable-experiment=non-nullable n018.dart
Analyzing n018.dart...
No issues found!
> /usr/local/google/home/eernst/bin/dart-sdk-2.3.3-dev.0.0/bin/dartanalyzer --enable-experiment=non-nullable n018.dart
Analyzing n018.dart...
  error • The return type 'dynamic' isn't a 'Future<bool>', as defined by the method 'test1' at n018.dart:3:31 • return_of_invalid_type
1 error found.

One more thing happened in 2.8, because it now claims that the return type is bool in a situation where the declaration explicitly says Future<bool>.

[eernstg]

There is a clear issue with the wording of the error message here: The return type of test1 is not bool.

With respect to the treatment of returned expressions of type dynamic in an async function, note that some discussions are taking place here: dart-lang/language#914.

[stereotype441]

There is a clear issue with the wording of the error message here: The return type of test1 is not bool.

With respect to the treatment of returned expressions of type dynamic in an async function, note that some discussions are taking place here: dart-lang/language#914.

@eernstg @leafpetersen it sounds like the discussions taking place there are about the possibility of changing the specified return rules. Can you clarify whether @iarkh's code snippet above ought to produce a compile time error under the current rules? As I understand it, currently the analyzer produces an error, and the co19 tests expect no error, so we are seeing co19 failures. It would be nice if in the short term (while we wait for a decision on dart-lang/language#914), we could change either the analyzer implementation or the co19 tests to match the current spec. But it's not clear to me from the discussion here which is correct under the current spec rules.

Thanks!

[leafpetersen]

The analyzer behavior is correct. Section 9 of the spec says that it is a compile time error if:

The function is asynchronous, flatten(T) is not void, and it would
have been a compile-time error to declare the function with the body
async { return e; } rather than async => e

and the relevant passage of section 17.12 is:

 It is a compile-time
error if s is return e;, flatten(S) is not void, and Future<flatten(S)> is not
assignable to T

here flatten(S) is dynamic and Future<dynamic> is not assignable to Future<bool> when the nnbd flag is enabled.

If any changes are forthcoming in the spec we will open issues for implementation when needed.