Fix a bug where a double-configuration error wouldn't be emitted

[nex3]

Closes #2598

See sass/sass#4111
See sass/sass-spec#2061

[arikorn]

I am concerned that this PR would break existing codebases (see @nex3's justification for this PR in #2598). It is possible that my PR #2602 would mitigate these effects.

[arikorn]

Thank you for the update @nex3: this is great! It appears to resolve the examples we've discussed and to accomplish what I intended in PR #2602.

There is a minor issue if the variable is in an @use file instead of @forward . For example:

// App.scss (top-level Sass file)

@use 'scss/coreui2' with (
     $white: #aaa
) ;

// scss/coreui2.scss
@use "variables";
@forward "functions";

with the remaining files are as described in Issue #2598, and in https://github.com/arikorn/sass-bug-demo

The result is:

Error: This variable was not declared with !default in the @used module.
  ╷
9 │       $white: #aaa
  │       ^^^^^^^^^^^^
  ╵
  ..\dart-tests\App2.scss 9:7  root stylesheet

The error message is wrong ($white is declared with !default and it's in _variables.css) and could be probably be avoided if @use was treated similarly to @forward, i.e. don't cause an error if there are no remaining config variables.

Just to be clear: the only change here from the previous examples is that coreui2.scss loads variables.scss with @use instead of @forward

Note, though, that this is not a breaking change, since the same input causes an error when using the main branch as well.

I have other specific comments that I'll attach to the code review, but nothing bigger than this.

[arikorn]

Oh drat! It also fails the test you mention here, which can be rephrased as:

 // App.scss (top-level Sass file)

@use 'scss/coreui2' as _;
@use 'scss/coreui2' with (
     $white: #aaa
) ;

(Giving a "variable not declared with !default" error instead of "module was already loaded" error.)

Perhaps the answer is that this is a work-in-progress, and those unused functions will fix it?