Duplicate IDs with CheckboxSelectMultiple if ID passed to constructor

[lraucy]

When a CheckboxSelectMultiple widget is instantiated with attrs passed to the constructor, render() produces invalid HTML:
all the input tags use the same html ID.

Exemple:

from django.forms.widgets import CheckboxSelectMultiple
print CheckboxSelectMultiple(attrs={'id': 'abc'}).render('letters', list('ac'), choices=zip(list('abc'), list('ABC')))

Result:

<ul>
<li><label><input checked="checked" type="checkbox" id="abc" value="a" name="letters" /> A</label></li>
<li><label><input type="checkbox" id="abc" value="b" name="letters" /> B</label></li>
<li><label><input checked="checked" type="checkbox" id="abc" value="c" name="letters" /> C</label></li>
</ul>

[lraucy]

Patch ​https://github.com/django/django/pull/626

[Preston Holmes]

This is prob RFC but on my mobile so can't do a full verification, but looks good.

[Preston Holmes <preston@…>]

In 62f842e2e568c87387e3da3ab76600bd0aab3e95:

Fixed #19581 -- ensure unique html ids with CheckboxSelectMultiple widgets

ID check is now done the same way as MultipleHiddenInput.