fix corner case in arguments (#4967)

lib/compress.js

@@ -11543,15 +11543,15 @@ merge(Compressor.prototype, {
         }
         var parent = compressor.parent();
         var assigned = is_lhs(compressor.self(), parent);
-        var def, fn, fn_parent;
+        var def, fn, fn_parent, index;
         if (compressor.option("arguments")
             && expr instanceof AST_SymbolRef
             && is_arguments(def = expr.definition())
             && !expr.in_arg
             && prop instanceof AST_Number
+            && Math.floor(index = prop.value) == index
             && (fn = def.scope) === find_lambda()
             && fn.uses_arguments < (assigned ? 2 : 3)) {
-            var index = prop.value;
             if (parent instanceof AST_UnaryPrefix && parent.operator == "delete") {
                 if (!def.deleted) def.deleted = [];
                 def.deleted[index] = true;

test/compress/arguments.js

@@ -254,6 +254,25 @@ duplicate_argname: {
     expect_stdout: "bar 42 foo 42 bar"
 }
 
+fraction: {
+    options = {
+        arguments: true,
+        keep_fargs: false,
+        reduce_vars: true,
+    }
+    input: {
+        console.log(function() {
+            return arguments[0.3];
+        }("FAIL") || "PASS");
+    }
+    expect: {
+        console.log(function() {
+            return arguments[0.3];
+        }("FAIL") || "PASS");
+    }
+    expect_stdout: "PASS"
+}
+
 issue_3273: {
     options = {
         arguments: true,