fix corner case in `awaits` (#4740)

fixes #4738

1 files changed, 22 insertions, 3 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 9d390d09..20558ba8 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -7205,10 +7205,10 @@ merge(Compressor.prototype, {
         });
         def(AST_Await, function(compressor) {
             if (!compressor.option("awaits")) return this;
-            var exp = this.expression.drop_side_effect_free(compressor);
-            if (exp === this.expression) return this;
+            var exp = this.expression;
+            if (!is_primitive(compressor, exp)) return this;
             var node = this.clone();
-            node.expression = exp || make_node(AST_Number, this, { value: 0 });
+            node.expression = exp.drop_side_effect_free(compressor) || make_node(AST_Number, this, { value: 0 });
             return node;
         });
         def(AST_Binary, function(compressor, first_in_statement) {
@@ -9555,6 +9555,25 @@ merge(Compressor.prototype, {
             || node instanceof AST_Object;
     }
 
+    function is_primitive(compressor, node) {
+        if (node.is_constant()) return true;
+        if (node instanceof AST_Assign) return node.operator != "=" || is_primitive(compressor, node.right);
+        if (node instanceof AST_Binary) {
+            return !lazy_op[node.operator]
+                || is_primitive(compressor, node.left) && is_primitive(compressor, node.right);
+        }
+        if (node instanceof AST_Conditional) {
+            return is_primitive(compressor, node.consequent) && is_primitive(compressor, node.alternative);
+        }
+        if (node instanceof AST_Sequence) return is_primitive(compressor, node.tail_node());
+        if (node instanceof AST_SymbolRef) {
+            var fixed = node.fixed_value();
+            return fixed && is_primitive(compressor, fixed);
+        }
+        if (node instanceof AST_Template) return !node.tag || is_raw_tag(compressor, node.tag);
+        if (node instanceof AST_Unary) return true;
+    }
+
     function repeatable(compressor, node) {
         if (node instanceof AST_Dot) return repeatable(compressor, node.expression);
         if (node instanceof AST_Sub) {