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

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

in-input {a: b}

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

<===> _upstream.scss
in-upstream {@extend in-input}

<===> error
Error: The target selector was not found.
Use "@extend in-input !optional" to avoid this error.
  ,
1 | in-upstream {@extend in-input}
  |              ^^^^^^^^^^^^^^^^
  '
  _upstream.scss 1:14  root stylesheet