Fix #415

ecmascript/codegen/src/lib.rs

@@ -1636,8 +1636,22 @@ impl<'a> Emitter<'a> {
 
         keyword!("return");
         if let Some(ref arg) = node.arg {
+            let need_paren = if let Some(cmt) = self.comments {
+                !self.pos_of_leading_comments.contains(&arg.span().lo()) && {
+                    // see #415
+                    cmt.leading_comments(arg.span().lo()).is_some()
+                }
+            } else {
+                false
+            };
+            if need_paren {
+                punct!("(");
+            }
             space!();
-            emit!(arg)
+            emit!(arg);
+            if need_paren {
+                punct!(")");
+            }
         }
         semi!();
     }

ecmascript/parser/src/lexer/state.rs

@@ -246,6 +246,13 @@ impl State {
             .unwrap_or(false)
     }
 
+    pub fn last_was_return(&self) -> bool {
+        match self.token_type {
+            Some(TokenType::Keyword(Keyword::Return)) => true,
+            _ => false,
+        }
+    }
+
     pub fn last_was_tpl_element(&self) -> bool {
         match self.token_type {
             Some(TokenType::Template) => true,

ecmascript/parser/src/lexer/util.rs

@@ -205,7 +205,7 @@ impl<'a, I: Input> Lexer<'a, I> {
             false
         };
 
-        let is_for_next = self.state.had_line_break;
+        let is_for_next = self.state.had_line_break || self.state.last_was_return();
 
         while let Some(c) = self.cur() {
             if was_star && c == '/' {