Libsass: 具有复合“＆”选择器的Mixins使源映射无效

在一组特定条件下，使用&复合选择器（例如&.mod-enabled或&:after ）使sourcemap生成器生成带有负列索引的sourcemap。

条件是：

• 规则必须是使用上述父选择器的复合选择器
• 该规则必须位于mixin的根中
• mixin必须至少包含在文档中6次

当gulp-sass切换到node-sass版本4.0时，这为我弹出了，这将LibSass推到了3.4。 我可以制作的最小的可复制测试用例在这里可用。

样本错误：

Error: Invalid mapping: {"generated":{"line":1,"column":6},"source":"mixins.scss","original":{"line":3,"column":-4},"name":null}
    at SourceMapGenerator_validateMapping [as _validateMapping] (/Users/cepheus/break-gulp-sass/node_modules/source-map/lib/source-map-generator.js:277:13)
    at SourceMapGenerator_addMapping [as addMapping] (/Users/cepheus/break-gulp-sass/node_modules/source-map/lib/source-map-generator.js:101:12)
    at /Users/cepheus/break-gulp-sass/node_modules/concat-with-sourcemaps/index.js:60:28
    at Array.forEach (native)
    at SourceMapConsumer_eachMapping [as eachMapping] (/Users/cepheus/break-gulp-sass/node_modules/source-map/lib/source-map-consumer.js:155:14)
    at Concat.add (/Users/cepheus/break-gulp-sass/node_modules/concat-with-sourcemaps/index.js:58:18)
    at DestroyableTransform.bufferContents [as _transform] (/Users/cepheus/break-gulp-sass/node_modules/gulp-concat/index.js:68:12)
    at DestroyableTransform.Transform._read (/Users/cepheus/break-gulp-sass/node_modules/readable-stream/lib/_stream_transform.js:159:10)
    at DestroyableTransform.Transform._write (/Users/cepheus/break-gulp-sass/node_modules/readable-stream/lib/_stream_transform.js:147:83)
    at doWrite (/Users/cepheus/break-gulp-sass/node_modules/readable-stream/lib/_stream_writable.js:347:64)

测试用例的两部分显示，此问题已在commit de83d30中引入。