stub out flexbox grid variation via -flex

scss/mixins/_grid-framework.scss

@@ -22,7 +22,11 @@
     @include media-breakpoint-up($breakpoint) {
       // Work around cross-media @extend (https://github.com/sass/sass/issues/1050)
       %grid-column-float-#{$breakpoint} {
-        float: left;
+        @if $enable-flex {
+          // Do nothing
+        } @else {
+          float: left;
+        }
       }
       @for $i from 1 through $columns {
         .col-#{$breakpoint}-#{$i} {

scss/mixins/_grid.scss

@@ -11,6 +11,10 @@
 }
 
 @mixin make-row($gutter: $grid-gutter-width) {
+  @if $enable-flex {
+    display: flex;
+    flex-wrap: wrap;
+  }
   margin-left:  ($gutter / -2);
   margin-right: ($gutter / -2);
   @include clearfix();
@@ -18,14 +22,22 @@
 
 @mixin make-col($gutter: $grid-gutter-width) {
   position: relative;
-  float: left;
+  @if $enable-flex {
+    // Do nothing
+  } @else {
+    float: left;
+  }
   min-height: 1px;
   padding-left:  ($gutter / 2);
   padding-right: ($gutter / 2);
 }
 
 @mixin make-col-span($size, $columns: $grid-columns) {
-  width: percentage($size / $columns);
+  @if $enable-flex {
+    flex: 0 0 percentage($size / $columns);
+  } @else {
+    width: percentage($size / $columns);
+  }
 }
 
 @mixin make-col-offset($size, $columns: $grid-columns) {