Correct (and nicer), version:

//Line 513
if(keyValue==value) {
  out << 'selected="selected" '
}

changed to

//Line 513
if(attrs.multiple) {
  if((value instanceof ArrayList && value.contains(keyValue)) || (!(value instanceof ArrayList) && keyValue == value)) {
    out << 'selected="selected" '
  }
}
else {
  if(keyValue==value) {
    out << 'selected="selected" '
  }
}

Please consider adding this in, either with this patch directly or by similar code. It's a pretty easy thing to add, and will make using the g:select tag much more robust for lists.

<g:select from="[1, 2, 3, 4]" value="[2, 4]" />

you will see that you cannot simply do a toString() call on the key value since value is a list of numbers not a list of Strings.

The current fix for GRAILS-1250 of converting the value and key to strings before the comparison works for numbers, but will it work for all object types? There is no guarantee that toString().equals() will have the same behaviour as equals(). This may be more of a theoretical problem than otherwise, but it still worries me.

Note that I have no objection to the requested functionality, I simply have concerns about how best to implement it .

I wasn't suggesting that you didn't want the functionality, but perhaps text only doesn't always get across the whole tone of a post.

Thanks for the attention on this one!

void testSimpleSelect() {
        def template = '<g:select name="foo" from="[1,2,3]" value="1" />'

assertOutputEquals('''<select name="foo" id="foo" >
<option value="1" selected="selected" >1</option>
<option value="2" >2</option>
<option value="3" >3</option>
</select>''', template)

    }


    void testMultiSelect() {
        def template = '<g:select name="foo" from="[1,2,3]" value="[2,3]" />'

assertOutputEquals('''<select name="foo" id="foo" multiple="true" >
<option value="1" >1</option>
<option value="2" selected="selected" >2</option>
<option value="3" selected="selected" >3</option>
</select>''', template)

    }

Note you dont have to set multiple=true it just assumes that the case if the value is a collection (i don't see what else it could be other than a multi select in this case)