title: Webpack silently not building nested scss
body: <p>I have a fairly simple webpack file which takes some typescript + scss and processes it. 
  I noticed that when I have nested scss, webpack will not transpile it into css at all. I have no data of it in the transpiled file. But when I have un-nested scss, everything works as expected. </p>
  
  <p>As an example, this is the scss I have that does not build with my webpack config. </p>
  
  <pre><code>.my-gallery {
    border: 2px solid red;
    &amp;-img {
      border: 2px solid blue;
    }
  }
  
  </code></pre>
  
  <p>However, when I have my scss like this, everything works as expected. </p>
  
  <pre><code>
      .my-gallery {
        border: 2px solid blue;
      }
  
      .my-gallery-img {
        border: 2px solid red;
      }
  
  </code></pre>
  
  <p>What could I be doing wrong in my webpack config? Below I have attached my current configuration and my dev dependencies. </p>
  
  <pre class="lang-js prettyprint-override"><code>const path = require("path");
  const MiniCssExtractPlugin = require("mini-css-extract-plugin");
  
  module.exports = {
    entry: ["./app/main.ts"],
    watch: true,
    mode: "production",
    output: {
      filename: "[name].js",
      path: path.resolve(__dirname, "static")
    },
    module: {
      rules: [
        {
          // Transform es6+ js &amp; ts into es5 js.
          test: [/.js$|.ts$/],
          exclude: /(node_modules)/,
          use: {
            loader: "babel-loader",
            options: {
              presets: ["@babel/preset-env", "@babel/typescript"]
            }
          }
        },
        {
          // Transform scss to css, minify it, apply autoprefixer via postcss
          test: [/.css$|.scss$/],
          use: [
            MiniCssExtractPlugin.loader,
            "css-loader",
            "sass-loader",
            "postcss-loader"
          ]
        }
      ]
    },
    resolve: {
      extensions: [".js", ".ts"]
    },
    plugins: [
      new MiniCssExtractPlugin({
        //Writes all styles to a single style.css file.
  
        filename: "css/[name].css"
      })
    ]
  };
  
  </code></pre>
  
  <pre><code>"devDependencies": {
      "@babel/core": "^7.5.5",
      "@babel/preset-env": "^7.5.5",
      "@babel/preset-typescript": "^7.3.3",
      "babel-loader": "^8.0.6",
      "autoprefixer": "^9.6.1",
      "webpack": "^4.36.1",
      "webpack-cli": "^3.3.6",
      "mini-css-extract-plugin": "^0.7.0",
      "node-sass": "^4.12.0",
      "css-loader": "^3.1.0",
      "postcss-loader": "^3.0.0",
      "sass-loader": "^7.1.0"
    }
  </code></pre>