Better messages for optimized index errors.

sdk/lib/_internal/compiler/js_lib/js_array.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of _interceptors;
 
 /**
  * The interceptor class for [List]. The compiler recognizes this
  * class as an interceptor, and changes references to [:this:] to
  * actually use the receiver of the method, which is generated as an extra
@@ skipping 557 matching lines @@
   Set<E> toSet() => new Set<E>.from(this);
 
   Iterator<E> get iterator => new ArrayIterator<E>(this);
 
   int get hashCode => Primitives.objectHashCode(this);
 
   int get length => JS('JSUInt32', r'#.length', this);
 
   void set length(int newLength) {
     checkGrowable('set length');
-    if (newLength is !int) throw new ArgumentError(newLength);
+    if (newLength is !int) throw new ArgumentError.value(newLength);

[Lasse Reichstein Nielsen, 2015/06/12 14:24:36]

consider passing "newLength" as second argument.

[sra1, 2015/06/12 22:29:04]

Done.

     if (newLength < 0) throw new RangeError.value(newLength);

[Lasse Reichstein Nielsen, 2015/06/12 14:24:36]

Consider using new RangeError.range(newLength, 0, null, "newLength").
That will document that the value must be non-negative (as is done by
RangeError.checkNotNegative).

[sra1, 2015/06/12 22:29:05]

I have done this for now.

The error feels wordy and indirect, and does not capture the max limit.

RangeError (newLength): Invalid value: Not greater than or equal to 0: -5

I'm considering removing the check and falling back on JS's error. This
less-informative message would permit more optimizations (e.g. generating JS 
a.length = b when all we know about b is that it is an integer).

RangeError: Invalid array length
    at JSArray.dart.JSArray.set$length (out.js:414:25)
    at dart.main (out.js:1170:27)


FWIW the VM's error is a bit misleading.

    [1].length = -3;

gives:
RangeError (index): Invalid value: Not in range 0..1, inclusive: -3
#0      List.[]= (dart:core-patch/growable_array.dart:138)
#1      List.length= (dart:core-patch/growable_array.dart:126)
#2      test (file:///home/sra/play1/bug6i6b.dart:8:7)

     JS('void', r'#.length = #', this, newLength);
   }
 
   E operator [](int index) {
-    if (index is !int) throw new ArgumentError(index);
-    if (index >= length || index < 0) throw new RangeError.value(index);
+    if (index is !int) throw diagnoseIndexError(this, index);
+    if (index >= length || index < 0) throw diagnoseIndexError(this, index);
     return JS('var', '#[#]', this, index);
   }
 
   void operator []=(int index, E value) {
     checkMutable('indexed set');
-    if (index is !int) throw new ArgumentError(index);
-    if (index >= length || index < 0) throw new RangeError.value(index);
+    if (index is !int) throw diagnoseIndexError(this, index);
+    if (index >= length || index < 0) throw diagnoseIndexError(this, index);
     JS('void', r'#[#] = #', this, index, value);
   }
 
   Map<int, E> asMap() {
     return new ListMapView<E>(this);
   }
 }
 
 /**
  * Dummy subclasses that allow the backend to track more precise
@@ skipping 37 matching lines @@
 
     if (_index >= length) {
       _current = null;
       return false;
     }
     _current = _iterable[_index];
     _index++;
     return true;
   }
 }