<===> input.scss
.z-depth-1 {
  box-shadow: 0 2px 5px 0 rgba(0, 0, 0, 0.16), 0 2px 10px 0 rgba(0, 0, 0, 0.12);
}

.z-depth-1-half {
  box-shadow: 0 5px 11px 0 rgba(0, 0, 0, 0.18), 0 4px 15px 0 rgba(0, 0, 0, 0.15);
}

.btn {
  @extend .z-depth-1;

  &:hover {
    @extend .z-depth-half-1;
  }
}

<===> error
Error: ".btn:hover" failed to @extend ".z-depth-half-1".
       The selector ".z-depth-half-1" was not found.
       Use "@extend .z-depth-half-1 !optional" if the extend should be able to fail.
        on line 13 of input.scss
  Use --trace for backtrace.

<===> error-libsass
Error: The target selector was not found.
       Use "@extend .z-depth-half-1 !optional" to avoid this error.
        on line 13 of input.scss
>>     @extend .z-depth-half-1;
   ------------^

<===> error-dart-sass
Error: The target selector was not found.
Use "@extend .z-depth-half-1 !optional" to avoid this error.
   ,
13 |     @extend .z-depth-half-1;
   |     ^^^^^^^^^^^^^^^^^^^^^^^
   '
  input.scss 13:5  root stylesheet