Merge branch 'issue19217' of https://github.com/alberto/bootstrap into alberto-issue19217

diff --cc scss/_variables.scss
index 4b5af26ad0cd2adf2d51dd635e9f895583ec6236,8e9bd61284c0e380a58b40c6c9a18805d16d28de..1a68a1fbf1fb84723e77e1f064b523a978011586
--- 1/scss/_variables.scss
--- 2/scss/_variables.scss
+++ b/scss/_variables.scss
@@@ -58,22 -29,14 +58,31 @@@
    }
  }
  
- /// Replace `$search` with `$replace` in `$string`
- /// @author Hugo Giraudel
- /// @param {String} $string - Initial string
- /// @param {String} $search - Substring to replace
- /// @param {String} $replace ('') - New value
- /// @return {String} - Updated string
++// Replace `$search` with `$replace` in `$string`
++// @author Hugo Giraudel
++// @param {String} $string - Initial string
++// @param {String} $search - Substring to replace
++// @param {String} $replace ('') - New value
++// @return {String} - Updated string
 +@function str-replace($string, $search, $replace: "") {
 +  $index: str-index($string, $search);
 +
 +  @if $index {
 +    @return str-slice($string, 1, $index - 1) + $replace + str-replace(str-slice($string, $index + str-length($search)), $search, $replace);
 +  }
 +
 +  @return $string;
 +}
 +
+ @mixin _assert-starts-at-zero($map) {
+   $values: map-values($map);
+   $first-value: nth($values, 1);
+   @if $first-value != 0 {
+     @warn "First breakpoint in `$grid-breakpoints` must start at 0, but starts at #{$first-value}.";
+   }
+ }
+ 
++
  // General variable structure
  //
  // Variable format should follow the `$component-modifier-state-property` order.
@@@ -172,9 -134,10 +181,10 @@@ $grid-breakpoints: 
    xl: 1200px
  ) !default;
  @include _assert-ascending($grid-breakpoints, "$grid-breakpoints");
+ @include _assert-starts-at-zero($grid-breakpoints);
  
  
 -// Grid containers
 +// 7. Grid containers
  //
  // Define the maximum width of `.container` for different screen sizes.