fix corner case in `collapse_vars` (#4199)

fixes #4197

1 files changed, 6 insertions, 2 deletions

diff --git a/lib/compress.js b/lib/compress.js
index c4012031..fc65e426 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1599,9 +1599,13 @@ merge(Compressor.prototype, {
                 }
                 if (node instanceof AST_SymbolRef) {
                     if (symbol_in_lvalues(node, parent)) {
-                        return !parent || parent.operator != "=" || parent.left !== node;
+                        return !(parent instanceof AST_Assign && parent.operator == "=" && parent.left === node);
                     }
-                    return side_effects && may_modify(node);
+                    if (side_effects && may_modify(node)) return true;
+                    var def = node.definition();
+                    return (in_try || def.scope.resolve() !== scope) && !all(def.orig, function(sym) {
+                        return !(sym instanceof AST_SymbolConst);
+                    });
                 }
                 if (node instanceof AST_This) return symbol_in_lvalues(node, parent);
                 if (node instanceof AST_VarDef) {