fix corner cases in `dead_code` & `reduce_vars` (#4213)

fixes #4212

1 files changed, 6 insertions, 3 deletions

diff --git a/lib/compress.js b/lib/compress.js
index e36fcaa7..5dd10c6c 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -485,9 +485,10 @@ merge(Compressor.prototype, {
         }
 
         function safe_to_assign(tw, def, declare) {
-            if (def.fixed === undefined) return declare || all(def.orig, function(sym) {
+            if (!(declare || all(def.orig, function(sym) {
                 return !(sym instanceof AST_SymbolConst);
-            });
+            }))) return false;
+            if (def.fixed === undefined) return true;
             if (def.fixed === null && def.safe_ids) {
                 def.safe_ids[def.id] = false;
                 delete def.safe_ids;
@@ -8557,7 +8558,9 @@ merge(Compressor.prototype, {
                         ]).optimize(compressor);
                     }
                 }
-            } else if (self.left instanceof AST_SymbolRef) {
+            } else if (self.left instanceof AST_SymbolRef && all(self.left.definition().orig, function(sym) {
+                return !(sym instanceof AST_SymbolConst);
+            })) {
                 var parent;
                 if (self.operator == "=" && self.left.equivalent_to(self.right)
                     && !((parent = compressor.parent()) instanceof AST_UnaryPrefix && parent.operator == "delete")) {