Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.1)
// Compass (v0.13.alpha.7)
// ----

$base-font-size: 16px;

@function em($px, $base: $base-font-size) {
  @return ($px / $base) * 1em;
}

$_var1: em(13px);

// This was giving an error
.foo {
    font-size: $_var1;
    line-height: (2 * $_var1);
}

// This was not.
.foo {
    font-size: $_var1;
    line-height: em(13px) * 2;
}

SassMeister-output.css

.foo {
  font-size: 0.8125em;
  line-height: 1.625em;
}

.foo {
  font-size: 0.8125em;
  line-height: 1.625em;
}