Applying darken function gives different result in libsass 3.5.0 and sass 3.4.21

[40in]

Here is the full Gist of the range colors #FFFFFF - #EEEEEE: libsas and sass versions

The same example without darken function gives all white, thus expected result here is the same output color. In fact, we have 2 different values going back and forth.

input.scss

test {
  border-top-color: darken(lighten(desaturate(#ffffff, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#fefefe, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#fdfdfd, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#fcfcfc, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#fbfbfb, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#fafafa, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#f9f9f9, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#f8f8f8, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#f7f7f7, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#f6f6f6, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#f5f5f5, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#f4f4f4, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#f3f3f3, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#f2f2f2, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#f1f1f1, 15.7%), 33.5%), 14.625%);
  border-top-color: darken(lighten(desaturate(#f0f0f0, 15.7%), 33.5%), 14.625%);
}

Actual results

libsass v3.5.2

test {
  border-top-color: #dadada;
  border-top-color: #dadada;
  border-top-color: #dadada;
  border-top-color: #b4ffff;
  border-top-color: #dadada;
  border-top-color: #b4ffff;
  border-top-color: #dadada;
  border-top-color: #b4ffff;
  border-top-color: #dadada;
  border-top-color: #b4ffff;
  border-top-color: #b4ffff;
  border-top-color: #dadada;
  border-top-color: #dadada;
  border-top-color: #b4ffff;
  border-top-color: #b4ffff;
  border-top-color: #dadada;
}

Expected result

sass v3.4.21

test {
  border-top-color: #dadada;
  border-top-color: #dadada;
  border-top-color: #dadada;
  ...
  border-top-color: #dadada; 
}

[mgreter]

Thanks, here a better test case showing the underlying issue:

test {
  $col: desaturate(#fcfcfc, 15.7%);
  test-col: desaturate(#fcfcfc, 15.7%);
  test-hue: hue(lighten($col, 33.5%));
  test-sat: saturation(lighten($col, 33.5%));
  test-lig: lightness(lighten($col, 33.5%));
  test-hsl: darken(hsl(0deg, 0%, 100%), 14.625%);
  combined: darken(lighten($col, 33.5%), 14.625%);
}

Ruby Sass:

test {
  test-col: #fcfcfc;
  test-hue: 0deg;
  test-sat: 0%;
  test-lig: 100%;
  test-hsl: #dadada;
  combined: #dadada; }

LibSass:

test {
  test-col: #fcfcfc;
  test-hue: 180deg;
  test-sat: 100%;
  test-lig: 100%;
  test-hsl: #dadada;
  combined: #b4ffff; }