bootstrap/scss/utilities/_float.scss

@each $breakpoint in map-keys($grid-breakpoints) {
  @include media-breakpoint-up($breakpoint) {
    $min: breakpoint-min($breakpoint, $grid-breakpoints);

    @if $min {
      // everything else
      @media (min-width: $min) {
        .float-#{$breakpoint}-left { @include float-left; }
        .float-#{$breakpoint}-right { @include float-right; }
        .float-#{$breakpoint}-none { @include float-none; }
      }
    } @else {
      // xs
      .float-left { @include float-left; }
      .float-right { @include float-right; }
      .float-none { @include float-none; }
    }
  }
}