Fix less &:extends when parsed with scss (#2034)

This is the only breakage that people ever reported with CSS so I'm happy adding one edge case like this.

Fixes #1967

src/printer-postcss.js

@@ -60,10 +60,21 @@ function genericPrint(path, options, print) {
       ]);
     }
     case "css-decl": {
+      // When the following less construct &:extend(.foo); is parsed with scss,
+      // it will put a space after `:` and break it. Ideally we should parse
+      // less files with less, but we can hardcode this to work with scss as
+      // well.
+      const isValueExtend =
+        n.value.type === "value-root" &&
+        n.value.group.type === "value-value" &&
+        n.value.group.group.type === "value-func" &&
+        n.value.group.group.value === "extend";
+
       return concat([
         n.raws.before.replace(/[\s;]/g, ""),
         n.prop,
-        ": ",
+        ":",
+        isValueExtend ? "" : " ",
         path.call(print, "value"),
         n.important ? " !important" : "",
         n.nodes

tests/css_extend/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,28 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`extend.css 1`] = `
+.foobar {
+  &:extend(.foo);
+}
+
+.thing {
+  &:hover {
+    background-color: blue;
+    .thing-child {
+    }
+  }
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+.foobar {
+  &:extend(.foo);
+}
+
+.thing {
+  &:hover {
+    background-color: blue;
+    .thing-child {
+    }
+  }
+}
+
+`;

tests/css_extend/extend.css

@@ -0,0 +1,11 @@
+.foobar {
+  &:extend(.foo);
+}
+
+.thing {
+  &:hover {
+    background-color: blue;
+    .thing-child {
+    }
+  }
+}

tests/css_extend/jsfmt.spec.js

@@ -0,0 +1 @@
+run_spec(__dirname, { parser: "postcss" });