Cannot use other units than 'px' for $alert-padding variable

[webmasterlv]

In v4 alpha3:

If trying to override $alert-padding with something like
$alert-padding: 1rem
The Sass compilation fails with
Error: Incompatible units: 'px' and 'rem'.

Because in _alert.scss there's this
padding-right: ($alert-padding + 20px);

[tschallacka]

$alert-padding: 2em;
.foo {
   padding-right: calc( #{$alert-padding} + 20px  );
 } 

might be a nice solution.

[lucasgladding]

Is there a reason this can't be done the same as the close button on modals? Removing the styling for .alert-dismissible and offsetting the top margin gives the same appearance for single lines.

.alert .close {
    margin-top: -2px;
}

For multiple lines, text could appear below the button, which is the only reason I can think of for the alternative approach.

[mdo]

Addressed with #20625.