AsyncIterator#return not called when error thrown in for await of loop

[trxcllnt]

The AsyncIterator#return method should be called by closure-compiler's async iteration polyfills, but doesn't seem to be (compiling with advanced opts):

$ node test.js 
> next: 0
> next: 1
> next: 2
> return called
> catch block
> finally block
all done

$ node test.min.js 
> next: 0
> next: 1
> next: 2
> catch block
> finally block
all done

test.js:

test().then(() => console.log('all done'));

async function test() {
  try {
    for await (const i of source()) {
      console.log(`> next: ${i}`);
      if (i === 2) {
        throw new Error('');
      }
    }
    console.log(`> done`);
  } catch (e) {
    console.log(`> catch block`);
  } finally {
    console.log(`> finally block`);
  }
}

function source() {
  return {
    i: 0,
    n: 3,
    [Symbol.asyncIterator]() { return this; },
    async next() {
      if (this.i < this.n) {
        return { done: false, value: this.i++ };
      }
      return { done: true };
    },
    async throw() {
      console.log(`> throw called`);
      return { done: true, value: undefined };
    },
    async return() {
      console.log(`> return called`);
      return { done: true, value: undefined };
    }
  };
}

test.min.js:

function $$jscomp$arrayIteratorImpl$$($array$jscomp$6$$) {
  var $index$jscomp$100$$ = 0;
  return function () {
    return $index$jscomp$100$$ < $array$jscomp$6$$.length ? { done: !1, value: $array$jscomp$6$$[$index$jscomp$100$$++], } : { done: !0 };
  };
}
var $$jscomp$defineProperty$$ = "function" == typeof Object.defineProperties ? Object.defineProperty : function ($target$jscomp$92$$, $property$jscomp$5$$, $descriptor$jscomp$1$$) {
  if ($target$jscomp$92$$ == Array.prototype || $target$jscomp$92$$ == Object.prototype) {
    return $target$jscomp$92$$;
  }
  $target$jscomp$92$$[$property$jscomp$5$$] = $descriptor$jscomp$1$$.value;
  return $target$jscomp$92$$;
};
function $$jscomp$getGlobal$$($passedInThis_possibleGlobals$$) {
  $passedInThis_possibleGlobals$$ = ["object" == typeof globalThis && globalThis, $passedInThis_possibleGlobals$$, "object" == typeof window && window, "object" == typeof self && self, "object" == typeof global && global,];
  for (var $i$jscomp$3$$ = 0; $i$jscomp$3$$ < $passedInThis_possibleGlobals$$.length; ++$i$jscomp$3$$) {
    var $maybeGlobal$$ = $passedInThis_possibleGlobals$$[$i$jscomp$3$$];
    if ($maybeGlobal$$ && $maybeGlobal$$.Math == Math) {
      return $maybeGlobal$$;
    }
  }
  throw Error("Cannot find global object");
}
var $$jscomp$global$$ = $$jscomp$getGlobal$$(this);
function $$jscomp$polyfill$$($property$jscomp$inline_10_split$jscomp$inline_7_target$jscomp$94$$, $impl$jscomp$inline_12_polyfill$jscomp$1$$) {
  if ($impl$jscomp$inline_12_polyfill$jscomp$1$$) {
    a: {
      var $obj$jscomp$inline_6$$ = $$jscomp$global$$;
      $property$jscomp$inline_10_split$jscomp$inline_7_target$jscomp$94$$ = $property$jscomp$inline_10_split$jscomp$inline_7_target$jscomp$94$$.split(".");
      for (var $i$jscomp$inline_8_orig$jscomp$inline_11$$ = 0; $i$jscomp$inline_8_orig$jscomp$inline_11$$ < $property$jscomp$inline_10_split$jscomp$inline_7_target$jscomp$94$$.length - 1; $i$jscomp$inline_8_orig$jscomp$inline_11$$++) {
        var $key$jscomp$inline_9$$ = $property$jscomp$inline_10_split$jscomp$inline_7_target$jscomp$94$$[$i$jscomp$inline_8_orig$jscomp$inline_11$$];
        if (!($key$jscomp$inline_9$$ in $obj$jscomp$inline_6$$)) {
          break a;
        }
        $obj$jscomp$inline_6$$ = $obj$jscomp$inline_6$$[$key$jscomp$inline_9$$];
      }
      $property$jscomp$inline_10_split$jscomp$inline_7_target$jscomp$94$$ = $property$jscomp$inline_10_split$jscomp$inline_7_target$jscomp$94$$[$property$jscomp$inline_10_split$jscomp$inline_7_target$jscomp$94$$.length - 1];
      $i$jscomp$inline_8_orig$jscomp$inline_11$$ = $obj$jscomp$inline_6$$[$property$jscomp$inline_10_split$jscomp$inline_7_target$jscomp$94$$];
      $impl$jscomp$inline_12_polyfill$jscomp$1$$ = $impl$jscomp$inline_12_polyfill$jscomp$1$$($i$jscomp$inline_8_orig$jscomp$inline_11$$);
      $impl$jscomp$inline_12_polyfill$jscomp$1$$ != $i$jscomp$inline_8_orig$jscomp$inline_11$$ && null != $impl$jscomp$inline_12_polyfill$jscomp$1$$ && $$jscomp$defineProperty$$($obj$jscomp$inline_6$$, $property$jscomp$inline_10_split$jscomp$inline_7_target$jscomp$94$$, { configurable: !0, writable: !0, value: $impl$jscomp$inline_12_polyfill$jscomp$1$$ });
    }
  }
}
$$jscomp$polyfill$$("Symbol", function ($orig$jscomp$1$$) {
  function $symbolPolyfill$$($opt_description$jscomp$2$$) {
    if (this instanceof $symbolPolyfill$$) {
      throw new TypeError("Symbol is not a constructor");
    }
    return new $SymbolClass$$($SYMBOL_PREFIX$$ + ($opt_description$jscomp$2$$ || "") + "_" + $counter$$++, $opt_description$jscomp$2$$);
  }
  function $SymbolClass$$($id$jscomp$5$$, $opt_description$jscomp$1$$) {
    this.$$jscomp$symbol$id_$ = $id$jscomp$5$$;
    $$jscomp$defineProperty$$(this, "description", { configurable: !0, writable: !0, value: $opt_description$jscomp$1$$ });
  }
  if ($orig$jscomp$1$$) {
    return $orig$jscomp$1$$;
  }
  $SymbolClass$$.prototype.toString = function () {
    return this.$$jscomp$symbol$id_$;
  };
  var $SYMBOL_PREFIX$$ = "jscomp_symbol_" + (1E9 * Math.random() >>> 0) + "_", $counter$$ = 0;
  return $symbolPolyfill$$;
});
$$jscomp$polyfill$$("Symbol.iterator", function ($orig$jscomp$2_symbolIterator$$) {
  if ($orig$jscomp$2_symbolIterator$$) {
    return $orig$jscomp$2_symbolIterator$$;
  }
  $orig$jscomp$2_symbolIterator$$ = Symbol("Symbol.iterator");
  for (var $arrayLikes$$ = "Array Int8Array Uint8Array Uint8ClampedArray Int16Array Uint16Array Int32Array Uint32Array Float32Array Float64Array".split(" "), $i$jscomp$6$$ = 0; $i$jscomp$6$$ < $arrayLikes$$.length; $i$jscomp$6$$++) {
    var $ArrayLikeCtor$$ = $$jscomp$global$$[$arrayLikes$$[$i$jscomp$6$$]];
    "function" === typeof $ArrayLikeCtor$$ && "function" != typeof $ArrayLikeCtor$$.prototype[$orig$jscomp$2_symbolIterator$$] && $$jscomp$defineProperty$$($ArrayLikeCtor$$.prototype, $orig$jscomp$2_symbolIterator$$, {
      configurable: !0, writable: !0, value: function () {
        return $$jscomp$iteratorPrototype$$($$jscomp$arrayIteratorImpl$$(this));
      }
    });
  }
  return $orig$jscomp$2_symbolIterator$$;
});
$$jscomp$polyfill$$("Symbol.asyncIterator", function ($orig$jscomp$3$$) {
  return $orig$jscomp$3$$ ? $orig$jscomp$3$$ : Symbol("Symbol.asyncIterator");
});
function $$jscomp$iteratorPrototype$$($iterator$jscomp$6_next$$) {
  $iterator$jscomp$6_next$$ = { next: $iterator$jscomp$6_next$$ };
  $iterator$jscomp$6_next$$[Symbol.iterator] = function () {
    return this;
  };
  return $iterator$jscomp$6_next$$;
}
function $$jscomp$asyncExecutePromiseGenerator$$($generator$jscomp$1$$) {
  function $passValueToGenerator$$($value$jscomp$99$$) {
    return $generator$jscomp$1$$.next($value$jscomp$99$$);
  }
  function $passErrorToGenerator$$($error$jscomp$2$$) {
    return $generator$jscomp$1$$.throw($error$jscomp$2$$);
  }
  return new Promise(function ($resolve$$, $reject$$) {
    function $handleGeneratorRecord$$($genRec$$) {
      $genRec$$.done ? $resolve$$($genRec$$.value) : Promise.resolve($genRec$$.value).then($passValueToGenerator$$, $passErrorToGenerator$$).then($handleGeneratorRecord$$, $reject$$);
    }
    $handleGeneratorRecord$$($generator$jscomp$1$$.next());
  });
}
function $$jscomp$AsyncIteratorFromSyncWrapper$$($iterator$jscomp$8$$) {
  this[Symbol.asyncIterator] = function () {
    return this;
  };
  this[Symbol.iterator] = function () {
    return $iterator$jscomp$8$$;
  };
  this.next = function ($param$jscomp$6$$) {
    return Promise.resolve($iterator$jscomp$8$$.next($param$jscomp$6$$));
  };
  void 0 !== $iterator$jscomp$8$$["throw"] && (this["throw"] = function ($param$jscomp$7$$) {
    return Promise.resolve($iterator$jscomp$8$$["throw"]($param$jscomp$7$$));
  });
  void 0 !== $iterator$jscomp$8$$["return"] && (this["return"] = function ($param$jscomp$8$$) {
    return Promise.resolve($iterator$jscomp$8$$["return"]($param$jscomp$8$$));
  });
}
function $__await$$module$node_modules$tslib$tslib_es6$$($v$jscomp$2$$) {
  return this instanceof $__await$$module$node_modules$tslib$tslib_es6$$ ? this : new $__await$$module$node_modules$tslib$tslib_es6$$($v$jscomp$2$$);
}
; $__await$$module$node_modules$tslib$tslib_es6$$.prototype[Symbol.toStringTag] = "__await";
Object.defineProperty($__await$$module$node_modules$tslib$tslib_es6$$, Symbol.hasInstance, {
  writable: !0, configurable: !0, value($inst$$) {
    return !(!$inst$$ || "__await" !== $inst$$[Symbol.toStringTag]);
  }
});
function $source$jscomp$17$$() {
  return {
    $i$: 0, n: 3, [Symbol.asyncIterator]() {
      return this;
    }, next() {
      const $$jscomp$async$this$$ = this;
      return $$jscomp$asyncExecutePromiseGenerator$$(function* () {
        return $$jscomp$async$this$$.$i$ < $$jscomp$async$this$$.n ? { done: !1, value: $$jscomp$async$this$$.$i$++ } : { done: !0 };
      }());
    }, throw() {
      return $$jscomp$asyncExecutePromiseGenerator$$(function* () {
        console.log("> throw called");
        return { done: !0, value: void 0 };
      }());
    }, return() {
      return $$jscomp$asyncExecutePromiseGenerator$$(function* () {
        console.log("> return called");
        return { done: !0, value: void 0 };
      }());
    }
  };
}
(function () {
  return $$jscomp$asyncExecutePromiseGenerator$$(function* () {
    try {
      var $$jscomp$forAwait$tempIterator0_JSCompiler_inline_result$jscomp$2$$, $iterable$jscomp$inline_14$$ = $source$jscomp$17$$(), $asyncIteratorFunction$jscomp$inline_15$$ = $iterable$jscomp$inline_14$$[Symbol.asyncIterator];
      if (void 0 !== $asyncIteratorFunction$jscomp$inline_15$$) {
        var $JSCompiler_temp$jscomp$17$$ = $asyncIteratorFunction$jscomp$inline_15$$.call($iterable$jscomp$inline_14$$);
      } else {
        var $iteratorFunction$jscomp$inline_21$$ = "undefined" != typeof Symbol && Symbol.iterator && $iterable$jscomp$inline_14$$[Symbol.iterator];
        var $JSCompiler_inline_result$jscomp$19$$ = $iteratorFunction$jscomp$inline_21$$ ? $iteratorFunction$jscomp$inline_21$$.call($iterable$jscomp$inline_14$$) : { next: $$jscomp$arrayIteratorImpl$$($iterable$jscomp$inline_14$$) };
        $JSCompiler_temp$jscomp$17$$ = new $$jscomp$AsyncIteratorFromSyncWrapper$$($JSCompiler_inline_result$jscomp$19$$);
      }
      for ($$jscomp$forAwait$tempIterator0_JSCompiler_inline_result$jscomp$2$$ = $JSCompiler_temp$jscomp$17$$; ;) {
        const $$jscomp$forAwait$tempResult0$$ = yield $$jscomp$forAwait$tempIterator0_JSCompiler_inline_result$jscomp$2$$.next();
        if ($$jscomp$forAwait$tempResult0$$.done) {
          break;
        }
        const $i$jscomp$18$$ = $$jscomp$forAwait$tempResult0$$.value;
        console.log(`> next: ${$i$jscomp$18$$}`);
        if (2 === $i$jscomp$18$$) {
          throw Error("");
        }
      }
      console.log("> done");
    } catch ($e$jscomp$18$$) {
      console.log("> catch block");
    } finally {
      console.log("> finally block");
    }
  }());
})().then(() => console.log("all done"));

[lauraharker]

Thanks for the report, definitely looks like a bug.

[rishipal]

Update: The fix for this caused runtime errors in a few internal tests. WIP fixing.

[rishipal]

Fixed with. d228c12