Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.0.rc.2)
// Compass (v1.0.0.alpha.17)
// ----

%some-extendable {
  width: 50%;
}
 
.some-class-name {
  @extend %some-extendable
}
 
// You could still do
.under-a-specific-context {
  %some-extendable {
    width: 35%;
  }
}

SassMeister-output.css

.some-class-name {
  width: 50%;
}

.under-a-specific-context .some-class-name {
  width: 35%;
}