fix corner case in `arguments` (#3330)

Track modifications to `arguments[i]` under Strict Mode.

fixes #3273

1 files changed, 21 insertions, 3 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 61c3a268..9109a696 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -497,6 +497,15 @@ merge(Compressor.prototype, {
             d.direct_access = true;
         }
 
+        function mark_assignment_to_arguments(node) {
+            if (!(node instanceof AST_Sub)) return;
+            var expr = node.expression;
+            var prop = node.property;
+            if (expr instanceof AST_SymbolRef && expr.name == "arguments" && prop instanceof AST_Number) {
+                expr.definition().reassigned = true;
+            }
+        }
+
         var suppressor = new TreeWalker(function(node) {
             if (!(node instanceof AST_Symbol)) return;
             var d = node.definition();
@@ -515,7 +524,10 @@ merge(Compressor.prototype, {
         def(AST_Assign, function(tw, descend, compressor) {
             var node = this;
             var sym = node.left;
-            if (!(sym instanceof AST_SymbolRef)) return;
+            if (!(sym instanceof AST_SymbolRef)) {
+                mark_assignment_to_arguments(sym);
+                return;
+            }
             var d = sym.definition();
             var safe = safe_to_assign(tw, d, sym.scope, node.right);
             d.assignments++;
@@ -758,7 +770,10 @@ merge(Compressor.prototype, {
             var node = this;
             if (node.operator != "++" && node.operator != "--") return;
             var exp = node.expression;
-            if (!(exp instanceof AST_SymbolRef)) return;
+            if (!(exp instanceof AST_SymbolRef)) {
+                mark_assignment_to_arguments(exp);
+                return;
+            }
             var d = exp.definition();
             var safe = safe_to_assign(tw, d, exp.scope, true);
             d.assignments++;
@@ -6325,7 +6340,10 @@ merge(Compressor.prototype, {
             var argname = fn.argnames[index];
             if (argname && compressor.has_directive("use strict")) {
                 var def = argname.definition();
-                if (!compressor.option("reduce_vars") || def.assignments || def.orig.length > 1) {
+                if (!compressor.option("reduce_vars")
+                    || expr.definition().reassigned
+                    || def.assignments
+                    || def.orig.length > 1) {
                     argname = null;
                 }
             } else if (!argname && !compressor.option("keep_fargs") && index < fn.argnames.length + 5) {