Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.4)
// Compass (v1.0.0.alpha.18)
// ----

// -----------------------------------------------------------------------------
// Introduction
// -----------------------------------------------------------------------------

// Here is hacky and experimental solution for cross-scopes extends
// ---
// It works by generating one placeholder per breakpoint
// instead of a single placeholder at root level as you'd normally do
// (in a fully automatic way of course)
// ---
// The API is quite simple.
// To create a placeholder, instead of doing:
//   %my-awesome-placeholder {} 
// You would do:
//   @include placeholder(my-awesome-placeholder)
// ---
// To extend an existing placeholder, instead of doing:
//   @extend %my-awesome-placeholder
// You would do:
//   @include _(my-awesome-placeholder)
// ---

// -----------------------------------------------------------------------------
// Set up
// -----------------------------------------------------------------------------

// A list of breakpoints
// When generating a placeholder,
// The same placeholder will be generated in every breakpoint
$breakpoints: (
  "small"  : 600px,
  "medium" : 900px,
  "large"  : 1200px
);

// -----------------------------------------------------------------------------
// Core
// -----------------------------------------------------------------------------

// Caching current breakpoint
// Not meant to be manually edited
$default-breakpoint: root;
$current-breakpoint: $default-breakpoint;

// Caching existing placeholders
// Not meant to be manually edited
$placeholders: ();

// The usual breakpoint mixin
// Except it updates the $current-breakpoint variable
// 1. If breakpoint name exists in map
// 2. Update $current-breakpoint
// 3. Open a media query
// 4. Let the user dump content
// 5. Then reset $current-breakpoint
// 6. If breakpoint name doesn't exist in map, warn the user
@mixin breakpoint($breakpoint) {
  $value: map-get($breakpoints, $breakpoint);
  
  @if $value != null {                                    // 1
    $current-breakpoint: $breakpoint         !global;     // 2
    @media (min-width: $value) { @content; }              // 3
    $current-breakpoint: $default-breakpoint !global;     // 5
  }
  
  @else {                                  
    @warn "Invalid breakpoint `#{$breakpoint}`.";         // 6
  }
}

// Generating placeholders
// actually generating one placeholder per breakpoint
// 1. If placeholder doesn't exist yet
// 2. Store the name
// 3. Looping through all the breakpoints
// 4. Opening a media query
// 5. Generating a placeholder at root level
// 6. With desired content
// 7. And dumping a placeholder out of any media query
// 8. If placeholder already exist, warn the user
@mixin placeholder($name) {
  @if not index($placeholders, $name) {                  // 1
    $placeholders: append($placeholders, $name) !global; // 2
    
    @at-root {
      @each $breakpoint, $value in $breakpoints {        // 3
        @media (min-width: $value) {                     // 4
          %#{$name}-#{$breakpoint} {                     // 5
            @content;                                    // 6
          }
        }
      }
    
      %#{$name}-#{$default-breakpoint} {                 // 7
        @content;
      }
    }
  }
  
  @else {
    @warn "Placeholder `#{$name}` already exists.";      // 8
  }
}

// Extend the accurate placeholder
// according to the current scope
// Basically instead of doing:
//    @extend %clear;
// You go:
//    @include _(clear);
// Not much longer, is it?
@mixin _($name) {
  @extend %#{$name}-#{$current-breakpoint} !optional;
}

// -----------------------------------------------------------------------------
// Generating a placeholder
// -----------------------------------------------------------------------------

// Creating placeholders
// A simple one in our case
// But it could have much more content
// ---
// Basically instead of doing:
//    %clear { ... }
// You go:
//    @include placeholder('clear') { ... }
@include placeholder('clear') {
  clear: both;
}

// -----------------------------------------------------------------------------
// Extending a placeholder
// -----------------------------------------------------------------------------

// At root level
.a {
  @include _(clear); // Same as @extend %clear
}

.b {
  @include _(clear);
}

// In a nested media query
.c {
  @include breakpoint(medium) {
    @include _(clear);
  }
}

// In an outer media query
@include breakpoint(medium) {
  .d {
    @include _(clear);
  }
}

// Both in a nested media query and at root level
.e {
  @include _(clear);

  @include breakpoint(large) {
    @include _(clear);
  }
}

// Have a look at the CSS output on the right.

SassMeister-output.css

@media (min-width: 900px) {
  .c, .d {
    clear: both;
  }
}
@media (min-width: 1200px) {
  .e {
    clear: both;
  }
}
.a, .b, .e {
  clear: both;
}