the behavior of selector.extend does not actually match the extend directive

[stof]

@extend forbids extending a compound selector. However, selector.extend happily accepts a:hover as its $extendee argument.
this makes the doc confusing, as it describes the function as following the same rules than @extend: https://sass-lang.com/documentation/modules/selector#extend

Thus, the doc about @extend says that @extend .a:hover should be replaced with @extend .a, :hover as it does the same (and indeed, in libsass, it does the same with a warning). However, passing .a, :hover or .a:hover to selector.extend produces a different behavior than passing .a, :hover to selector.extend.

.hoverlink { @extend .a, :hover }
foo .a:hover, bar .a, b :hover { text-decoration: underline }

f { 
    c: selector-extend("foo .a:hover, bar .a, b :hover", ".a, :hover", ".hoverlink") 
}

Here is the output with dart-sass 1.32.2:

foo .a:hover, foo .hoverlink, bar .a, bar .hoverlink, b :hover, b .hoverlink {
  text-decoration: underline;
}

f {
  c: foo .a:hover, foo .hoverlink, bar .a, b :hover;
}

[stof]

note that libsass gets the output right when passing .a, :hover to selector-extend (i.e. it handles selector lists of simple selectors properly) but does not produce the same output when passing the compound selector .a:hover

[stof]

Looking at the code, I found ExtendMode which describes that @extend and selector-extend don't actually run the same logic (they use different modes). So this might be a documentation issue rather than a bug in dart-sass then.

[nex3]

This looks like a bug to me.

Looking at the code, I found ExtendMode which describes that @extend and selector-extend don't actually run the same logic (they use different modes). So this might be a documentation issue rather than a bug in dart-sass then.

The ExtendMode is different only because all of the target selectors in selector-extend() are required to be used. Otherwise, the semantics should be identical.

[nex3]

Looking at this further:

• The current behavior of $extendee: ".a:hover" is intentional. It matches the behavior of LibSass and the historical behavior of Ruby Sass. It's essentially a holdover from allowing compound selectors in @extend, and I'm not sure why we explicitly replicated it in Dart Sass. We should probably deprecate it at some point. I've filed Forbid compound selector extensions in selector functions sass#3089 to track that.

• The behavior of $extendee: ".a, :hover" is clearly a bug that I'll see if I can go ahead and fix.

The ExtendMode is different only because all of the target selectors in selector-extend() are required to be used. Otherwise, the semantics should be identical.

I was wrong about this—ExtendMode does explicitly enable the behavior you're describing.