enhance `collapse_vars` (#4117)

2 files changed, 9 insertions, 4 deletions

diff --git a/lib/compress.js b/lib/compress.js
index f19832e1..89d698da 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1458,9 +1458,15 @@ merge(Compressor.prototype, {
             }
 
             function handle_custom_scan_order(node, tt) {
+                // Scan object only in a for-in statement
+                if (node instanceof AST_ForIn) {
+                    node.object = node.object.transform(tt);
+                    abort = true;
+                    return node;
+                }
                 // Skip (non-executed) functions
                 if (node instanceof AST_Scope) return node;
-                // Scan case expressions first in a switch statement
+                // Scan first case expression only in a switch statement
                 if (node instanceof AST_Switch) {
                     node.expression = node.expression.transform(tt);
                     for (var i = 0; !abort && i < node.body.length; i++) {
@@ -1493,7 +1499,7 @@ merge(Compressor.prototype, {
                 }
                 if (node instanceof AST_Debugger) return true;
                 if (node instanceof AST_Defun) return funarg && lhs.name === node.name.name;
-                if (node instanceof AST_IterationStatement) return !(node instanceof AST_For);
+                if (node instanceof AST_DWLoop) return true;
                 if (node instanceof AST_LoopControl) return true;
                 if (node instanceof AST_Try) return true;
                 if (node instanceof AST_With) return true;
diff --git a/test/compress/drop-unused.js b/test/compress/drop-unused.js
index 96273a8d..3e2b54f2 100644
--- a/test/compress/drop-unused.js
+++ b/test/compress/drop-unused.js
@@ -2181,8 +2181,7 @@ issue_3515_1: {
     expect: {
         var c = 0;
         (function() {
-            this[c++] = 0;
-            for (var key20 in !0);
+            for (var key20 in !(this[c++] = 0));
         })();
         console.log(c);
     }