<===> README.md
`@forward` should handle extensions in the same way as `@use`. We assume that
they use shared infrastructure, though, and as such only test basic cases.

<===>
================================================================================
<===> upstream/input.scss
@use "midstream";

in-input {@extend in-upstream}

<===> upstream/_midstream.scss
@forward "upstream";

<===> upstream/_upstream.scss
in-upstream {a: b}

<===> upstream/output.css
in-upstream, in-input {
  a: b;
}

<===>
================================================================================
<===> forward_into_use/input.scss
@use "midstream";

in-input {@extend in-used}

<===> forward_into_use/_midstream.scss
@forward "forwarded";

<===> forward_into_use/_forwarded.scss
@use "used";

<===> forward_into_use/_used.scss
in-used {a: b}

<===> forward_into_use/output.css
in-used, in-input {
  a: b;
}

<===>
================================================================================
<===> forward_into_import/input.scss
@use "midstream";

in-input {@extend in-imported}

<===> forward_into_import/_midstream.scss
@forward "forwarded";

<===> forward_into_import/_forwarded.scss
@import "imported";

<===> forward_into_import/_imported.scss
in-imported {a: b}

<===> forward_into_import/output.css
in-imported, in-input {
  a: b;
}