Don't lowercase &class in SCSS/Less selectors (#2834)

Fixes #2830

src/printer-postcss.js

@@ -217,7 +217,12 @@ function genericPrint(path, options, print) {
       return adjustStrings(n.value, options);
     }
     case "selector-tag": {
-      return maybeToLowerCase(n.value);
+      const parent = path.getParentNode();
+      const index = parent.nodes.indexOf(n);
+      const previous = index > 0 ? parent.nodes[index - 1] : null;
+      return previous && previous.type === "selector-nesting"
+        ? n.value
+        : maybeToLowerCase(n.value);
     }
     case "selector-id": {
       return concat(["#", n.value]);

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

@@ -50,6 +50,10 @@ a[HREF=KeepAttrValue]:HOVER::FIRST-letter,
       prop: val;
     }
 
+    &Keep & NoKeep {
+      prop: val;
+    }
+
     @KeepDetachedRuleset();
     @KeepDetachedRuleset ( /* comment */  );
 
@@ -123,6 +127,10 @@ a[href="KeepAttrValue"]:hover::first-letter,
     prop: val;
   }
 
+  &Keep & nokeep {
+    prop: val;
+  }
+
   @KeepDetachedRuleset();
   @KeepDetachedRuleset();
 
@@ -208,6 +216,10 @@ a[HREF=KeepAttrValue]:HOVER::FIRST-letter,
       prop: val;
     }
 
+    &Keep & NoKeep {
+      prop: val;
+    }
+
     BACKGROUND: {
       COLOR: RED;
     }
@@ -284,6 +296,10 @@ a[href="KeepAttrValue"]:hover::first-letter,
     prop: val;
   }
 
+  &Keep & nokeep {
+    prop: val;
+  }
+
   background: {
     color: RED;
   }

tests/css_case/case.less

@@ -47,6 +47,10 @@ a[HREF=KeepAttrValue]:HOVER::FIRST-letter,
       prop: val;
     }
 
+    &Keep & NoKeep {
+      prop: val;
+    }
+
     @KeepDetachedRuleset();
     @KeepDetachedRuleset ( /* comment */  );
 

tests/css_case/case.scss

@@ -50,6 +50,10 @@ a[HREF=KeepAttrValue]:HOVER::FIRST-letter,
       prop: val;
     }
 
+    &Keep & NoKeep {
+      prop: val;
+    }
+
     BACKGROUND: {
       COLOR: RED;
     }