Should await work in non-async methods?

[DartBot]

This issue was originally filed by sas...@chromium.org

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Dart currently does not recognize 'await' unless it is used inside an async method:

  $ cat foo.dart
  import 'dart:async';
  String foo() async => 'foo';
  void main() { print(await foo()); }

  $ dart --enable-async foo.dart
  'file:///Users/sascha/src/dart_async/foo.dart': error: line 3 pos 27: ')' expected
  void main() { print(await foo()); }
                            ^
When main() gets declared as async, Dart will execute the code just fine. So it seems that the 'await' statement currently gets only recognized when used inside an async method. Is this intentional? Arguably, the point of the 'await' statement is allowing synchronous code to wait for async results (futures). So, wouldn't the main utility of the 'await' statement be inside methods that are not declared async?

If the reason for the current behavior was backwards compatibility, couldn't the language feature get triggered by importing 'dart:async'? However, in case there's actually a deeper reason why 'await' can/should not work from inside non-async methods, it might be good to change the parser so it emits a better error message.

  $ dart --version
  Dart VM version: 1.8.5 (Tue Jan 13 13:04:11 2015) on "macos_ia32"

[lrhn]

Yes, "await" is only allowed inside an async or async* function.
I agree that the error message could be more informative, so I'll make this an enhancement request for a better error message from the VM.

An "await" expression does not let synchronous code wait for asynchronous code - that is simply not possible in Dart as it currently works. Only asynchronous code can wait, because the act of waiting is what means that it's not synchronous.

Backwards compatibility is an issue too, and merely importing "dart:async" doesn't mark anything as being new code. There might still be code in existing libraries importing "dart:async" that uses "await" as a variable.

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Removed Type-Defect label.
Added Type-Enhancement, Area-VM, Triaged labels.

[DartBot]

This comment was originally written by @mhausner

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As discussed in other issues, the VM compiler should not be expected to analyze illegal code and figure out if it were legal in a different context. 'await' is a normal identifier with no magic meaning in non-async code. Even though for a human reader, it's quite clear what await foo() means (or was meant to mean), the compiler sees this just like xyz abc(), which is clearly illegal.

The proper way to fix this is to make await a keyword in all dart code, not just async functions. That would be a language enhancement issue.

Meanwhile, the Analyzer is maybe a better place to warn about this.

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Added Duplicate label.
Marked as being merged into #22625.