VM crash on "await x++".

[lrhn]

Example code:
  main() async {
    var x = 0;
    await x++;
  }
yields:

runtime/vm/scopes.h:62: error: expected: HasIndex()
Aborted (core dumped)

[iposva-google]

cc @crelier.
Set owner to @mhausner.
Added this to the 1.9 milestone.
Removed Priority-Unassigned label.
Added Priority-High, Accepted labels.

[DartBot]

This comment was originally written by @mhausner

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The VM clearly should not crash.

What does await x++ mean? is it the same as x++; await x?

[lrhn]

  await x++
should be the same as
  await (x++)
or
  var tmp = x++;
  await tmp;
since the grammar is equivalent to :
  unaryExpression : ... | 'await' unaryExpression | postfixExpression | ...
and postfixExpression produces "x++".

[iposva-google]

Adding Gilad for the interpretation of the language spec. Thanks!

---

cc @gbracha.

[gbracha]

It is the same as await (x++);
which in turn means

await ( (){var r = v; v= r + 1; return r;}() );

[lrhn]

It no longer crashes, but it fails to work. The code
    f() async {
      var x = 42;
      return await x++;
    }
gives:

ERROR: await expression unary postfix operator
  Test failed: Caught Closure call with mismatched arguments: function '+'
  
  NoSuchMethodError: incorrect number of arguments passed to method named '+'
  Receiver: Closure: ([dynamic, dynamic, dynamic]) => dynamic
  Tried calling: +(1)
  Found: +(:async_result, :async_error_param, :async_stack_trace_param)
  dart:core-patch/object_patch.dart 42:35 Object._noSuchMethod
  dart:core-patch/object_patch.dart 45:25 Object.noSuchMethod
  ../tests/language/async_await_test.dart 1549:22 main.<fn>.<fn>.f.<async>

[DartBot]

This comment was originally written by @mhausner

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Nothing has changed for me. I still get the same assertion error. It's not an easy fix. The problem are LetNode ast nodes, which in at least two ways do not fit well into the ast transformation we do for await.

[lrhn]

ACK, I was running a non-debug build, hence the missing C++ assertion.

[DartBot]

This comment was originally written by @mhausner

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I have a fix for this.

---

Added Started label.

[DartBot]

This comment was originally written by @mhausner

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Fixed in r44337

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Added Fixed label.