Add add and subtract function

5 files changed, 97 insertions, 22 deletions

diff --git a/scss/_functions.scss b/scss/_functions.scss
index 930b7f8972..fc789ac258 100644
--- a/scss/_functions.scss
+++ b/scss/_functions.scss
@@ -95,3 +95,40 @@
 
   @return mix($color-base, $color, $level * $theme-color-interval);
 }
+
+// Return valid calc
+@function add($value1, $value2, $return-calc: true) {
+  @if $value1 == null {
+    @return $value2;
+  }
+
+  @if $value2 == null {
+    @return $value1;
+  }
+
+  @if type-of($value1) == number and type-of($value2) == number and comparable($value1, $value2) {
+    @return $value1 + $value2;
+  }
+
+  @return if($return-calc == true, calc(#{$value1} + #{$value2}), #{$value1} + #{$value2});
+}
+
+@function subtract($value1, $value2, $return-calc: true) {
+  @if $value1 == null and $value2 == null {
+    @return null;
+  }
+
+  @if $value1 == null {
+    @return -$value2;
+  }
+
+  @if $value2 == null {
+    @return $value1;
+  }
+
+  @if type-of($value1) == number and type-of($value2) == number and comparable($value1, $value2) {
+    @return $value1 - $value2;
+  }
+
+  @return if($return-calc == true, calc(#{$value1} - #{$value2}), #{$value1} - #{$value2});
+}
diff --git a/scss/_modal.scss b/scss/_modal.scss
index bc08617c95..9a036883a0 100644
--- a/scss/_modal.scss
+++ b/scss/_modal.scss
@@ -52,10 +52,10 @@
 
 .modal-dialog-scrollable {
   display: flex; // IE10/11
-  max-height: calc(100% - #{$modal-dialog-margin * 2});
+  max-height: subtract(100%, $modal-dialog-margin * 2);
 
   .modal-content {
-    max-height: calc(100vh - #{$modal-dialog-margin * 2}); // IE10/11
+    max-height: subtract(100vh, $modal-dialog-margin * 2); // IE10/11
     overflow: hidden;
   }
 
@@ -72,12 +72,12 @@
 .modal-dialog-centered {
   display: flex;
   align-items: center;
-  min-height: calc(100% - #{$modal-dialog-margin * 2});
+  min-height: subtract(100%, $modal-dialog-margin * 2);
 
   // Ensure `modal-dialog-centered` extends the full height of the view (IE10/11)
   &::before {
     display: block; // IE10
-    height: calc(100vh - #{$modal-dialog-margin * 2});
+    height: subtract(100vh, $modal-dialog-margin * 2);
     content: "";
   }
 
@@ -200,18 +200,18 @@
   }
 
   .modal-dialog-scrollable {
-    max-height: calc(100% - #{$modal-dialog-margin-y-sm-up * 2});
+    max-height: subtract(100%, $modal-dialog-margin-y-sm-up * 2);
 
     .modal-content {
-      max-height: calc(100vh - #{$modal-dialog-margin-y-sm-up * 2});
+      max-height: subtract(100vh, $modal-dialog-margin-y-sm-up * 2);
     }
   }
 
   .modal-dialog-centered {
-    min-height: calc(100% - #{$modal-dialog-margin-y-sm-up * 2});
+    min-height: subtract(100%, $modal-dialog-margin-y-sm-up * 2);
 
     &::before {
-      height: calc(100vh - #{$modal-dialog-margin-y-sm-up * 2});
+      height: subtract(100vh, $modal-dialog-margin-y-sm-up * 2);
     }
   }
 
diff --git a/scss/_popover.scss b/scss/_popover.scss
index c9b11e94e8..0ad76af3ee 100644
--- a/scss/_popover.scss
+++ b/scss/_popover.scss
@@ -39,7 +39,7 @@
   margin-bottom: $popover-arrow-height;
 
   > .arrow {
-    bottom: calc((#{$popover-arrow-height} + #{$popover-border-width}) * -1);
+    bottom: subtract(-$popover-arrow-height, $popover-border-width);
 
     &::before {
       bottom: 0;
@@ -59,7 +59,7 @@
   margin-left: $popover-arrow-height;
 
   > .arrow {
-    left: calc((#{$popover-arrow-height} + #{$popover-border-width}) * -1);
+    left: subtract(-$popover-arrow-height, $popover-border-width);
     width: $popover-arrow-height;
     height: $popover-arrow-width;
     margin: $popover-border-radius 0; // make sure the arrow does not touch the popover's rounded corners
@@ -82,7 +82,7 @@
   margin-top: $popover-arrow-height;
 
   > .arrow {
-    top: calc((#{$popover-arrow-height} + #{$popover-border-width}) * -1);
+    top: subtract(-$popover-arrow-height, $popover-border-width);
 
     &::before {
       top: 0;
@@ -114,7 +114,7 @@
   margin-right: $popover-arrow-height;
 
   > .arrow {
-    right: calc((#{$popover-arrow-height} + #{$popover-border-width}) * -1);
+    right: subtract(-$popover-arrow-height, $popover-border-width);
     width: $popover-arrow-height;
     height: $popover-arrow-width;
     margin: $popover-border-radius 0; // make sure the arrow does not touch the popover's rounded corners
diff --git a/scss/_variables.scss b/scss/_variables.scss
index be3a0f7985..8be63c6733 100644
--- a/scss/_variables.scss
+++ b/scss/_variables.scss
@@ -495,13 +495,13 @@ $input-plaintext-color:                 $body-color !default;
 
 $input-height-border:                   $input-border-width * 2 !default;
 
-$input-height-inner:                    calc(#{$input-line-height * 1em} + #{$input-padding-y * 2}) !default;
-$input-height-inner-half:               calc(#{$input-line-height * .5em} + #{$input-padding-y}) !default;
-$input-height-inner-quarter:            calc(#{$input-line-height * .25em} + #{$input-padding-y / 2}) !default;
+$input-height-inner:                    add($input-line-height * 1em, $input-padding-y * 2) !default;
+$input-height-inner-half:               add($input-line-height * .5em, $input-padding-y) !default;
+$input-height-inner-quarter:            add($input-line-height * .25em, $input-padding-y / 2) !default;
 
-$input-height:                          calc(#{$input-line-height * 1em} + #{$input-padding-y * 2} + #{$input-height-border}) !default;
-$input-height-sm:                       calc(#{$input-line-height-sm * 1em} + #{$input-btn-padding-y-sm * 2} + #{$input-height-border}) !default;
-$input-height-lg:                       calc(#{$input-line-height-lg * 1em} + #{$input-btn-padding-y-lg * 2} + #{$input-height-border}) !default;
+$input-height:                          add($input-line-height * 1em, add($input-padding-y * 2, $input-height-border, false)) !default;
+$input-height-sm:                       add($input-line-height-sm * 1em, add($input-btn-padding-y-sm * 2, $input-height-border, false)) !default;
+$input-height-lg:                       add($input-line-height-lg * 1em, add($input-btn-padding-y-lg * 2, $input-height-border, false)) !default;
 
 $input-transition:                      border-color .15s ease-in-out, box-shadow .15s ease-in-out !default;
 
@@ -766,7 +766,7 @@ $dropdown-bg:                       $white !default;
 $dropdown-border-color:             rgba($black, .15) !default;
 $dropdown-border-radius:            $border-radius !default;
 $dropdown-border-width:             $border-width !default;
-$dropdown-inner-border-radius:      calc(#{$dropdown-border-radius} - #{$dropdown-border-width}) !default;
+$dropdown-inner-border-radius:      subtract($dropdown-border-radius, $dropdown-border-width) !default;
 $dropdown-divider-bg:               $gray-200 !default;
 $dropdown-divider-margin-y:         $nav-divider-margin-y !default;
 $dropdown-box-shadow:               0 .5rem 1rem rgba($black, .175) !default;
@@ -831,7 +831,7 @@ $card-spacer-x:                     1.25rem !default;
 $card-border-width:                 $border-width !default;
 $card-border-radius:                $border-radius !default;
 $card-border-color:                 rgba($black, .125) !default;
-$card-inner-border-radius:          calc(#{$card-border-radius} - #{$card-border-width}) !default;
+$card-inner-border-radius:          subtract($card-border-radius, $card-border-width) !default;
 $card-cap-bg:                       rgba($black, .03) !default;
 $card-cap-color:                    null !default;
 $card-height:                       null !default;
@@ -881,7 +881,7 @@ $popover-max-width:                 276px !default;
 $popover-border-width:              $border-width !default;
 $popover-border-color:              rgba($black, .2) !default;
 $popover-border-radius:             $border-radius-lg !default;
-$popover-inner-border-radius:       calc(#{$popover-border-radius} - #{$popover-border-width}) !default;
+$popover-inner-border-radius:       subtract($popover-border-radius, $popover-border-width) !default;
 $popover-box-shadow:                0 .25rem .5rem rgba($black, .2) !default;
 
 $popover-header-bg:                 darken($popover-bg, 3%) !default;
@@ -953,7 +953,7 @@ $modal-content-bg:                  $white !default;
 $modal-content-border-color:        rgba($black, .2) !default;
 $modal-content-border-width:        $border-width !default;
 $modal-content-border-radius:       $border-radius-lg !default;
-$modal-content-inner-border-radius: calc(#{$modal-content-border-radius} - #{$modal-content-border-width}) !default;
+$modal-content-inner-border-radius: subtract($modal-content-border-radius, $modal-content-border-width) !default;
 $modal-content-box-shadow-xs:       0 .25rem .5rem rgba($black, .5) !default;
 $modal-content-box-shadow-sm-up:    0 .5rem 1rem rgba($black, .5) !default;
 
diff --git a/site/docs/4.3/getting-started/theming.md b/site/docs/4.3/getting-started/theming.md
index 198a55da3a..ecc5895b13 100644
--- a/site/docs/4.3/getting-started/theming.md
+++ b/site/docs/4.3/getting-started/theming.md
@@ -227,6 +227,44 @@ You can also specify a base color with our color map functions:
 
 We use the `escape-svg` function to escape the `<`, `>` and `#` characters for SVG background images. These characters need to be escaped to properly render the background images in IE.
 
+## Add and Subtract function
+
+We use the `add` and `subtract` functions instead of the CSS `calc` function. The primary purpose of these functions is to avoid errors when "unitless" 0 is given to the `calc` expression.
+
+Example where the calc is valid:
+
+{{< highlight scss >}}
+$border-radius: .25rem;
+$border-width: 1px;
+
+.element {
+  // Output calc(.25rem - 1px) is valid
+  border-radius: calc($border-radius - $border-width);
+}
+
+.element {
+  // Output the same calc(.25rem - 1px) as above
+  border-radius: subtract($border-radius, $border-width);
+}
+{{< /highlight >}}
+
+Example where the calc is invalid:
+
+{{< highlight scss >}}
+$border-radius: .25rem;
+$border-width: 0;
+
+.element {
+  // Output calc(.25rem - 0) is invalid
+  border-radius: calc($border-radius - $border-width);
+}
+
+.element {
+  // Output .25rem
+  border-radius: subtract($border-radius, $border-width);
+}
+{{< /highlight >}}
+
 ## Sass options
 
 Customize Bootstrap 4 with our built-in custom variables file and easily toggle global CSS preferences with new `$enable-*` Sass variables. Override a variable's value and recompile with `npm run test` as needed.