fix corner cases with asynchronous generators (#4642)

fixes #4641

1 files changed, 11 insertions, 2 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 8b5b020d..3d857190 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1307,6 +1307,10 @@ merge(Compressor.prototype, {
         return found;
     });
 
+    function in_async_generator(scope) {
+        return scope instanceof AST_AsyncGeneratorDefun || scope instanceof AST_AsyncGeneratorFunction;
+    }
+
     function find_scope(compressor) {
         var level = 0, node;
         while (node = compressor.parent(level++)) {
@@ -2987,7 +2991,9 @@ merge(Compressor.prototype, {
                 var stat = statements[i];
                 if (prev) {
                     if (stat instanceof AST_Exit) {
-                        stat.value = cons_seq(stat.value || make_node(AST_Undefined, stat)).transform(compressor);
+                        if (stat.value || !in_async_generator(scope)) {
+                            stat.value = cons_seq(stat.value || make_node(AST_Undefined, stat)).transform(compressor);
+                        }
                     } else if (stat instanceof AST_For) {
                         if (!(stat.init instanceof AST_Definitions)) {
                             var abort = false;
@@ -11045,7 +11051,10 @@ merge(Compressor.prototype, {
     });
 
     OPT(AST_Return, function(self, compressor) {
-        if (self.value && is_undefined(self.value, compressor)) {
+        if (compressor.option("side_effects")
+            && self.value
+            && is_undefined(self.value, compressor)
+            && !in_async_generator(compressor.find_parent(AST_Scope))) {
             self.value = null;
         }
         return self;