fix corner case in default_values (#4497)

fixes #4496

lib/compress.js

@@ -7692,7 +7692,9 @@ merge(Compressor.prototype, {
             return !(arg instanceof AST_Spread);
         })) return;
         var pos = 0, last = 0;
-        var drop_fargs = fn === exp && !fn.name && compressor.drop_fargs(fn, call) ? function(argname, arg) {
+        var is_iife = fn === exp && !fn.name;
+        var drop_defaults = is_iife && compressor.option("default_values");
+        var drop_fargs = is_iife && compressor.drop_fargs(fn, call) ? function(argname, arg) {
             if (!argname) return true;
             if (argname instanceof AST_DestructuredArray) {
                 return argname.elements.length == 0 && arg instanceof AST_Array;
@@ -7705,9 +7707,7 @@ merge(Compressor.prototype, {
         var side_effects = [];
         for (var i = 0; i < args.length; i++) {
             var argname = fn.argnames[i];
-            if (compressor.option("default_values")
-                && argname instanceof AST_DefaultValue
-                && args[i].is_defined(compressor)) {
+            if (drop_defaults && argname instanceof AST_DefaultValue && args[i].is_defined(compressor)) {
                 fn.argnames[i] = argname = argname.name;
             }
             if (!argname || "__unused" in argname) {

test/compress/default-values.js

@@ -1313,3 +1313,31 @@ issue_4485_3: {
     expect_stdout: true
     node_version: ">=6"
 }
+
+issue_4496: {
+    options = {
+        default_values: true,
+        unused: true,
+    }
+    input: {
+        (function f(a = 0) {
+            console.log(function(b) {
+                a && b();
+                return a;
+            }(f));
+        })(42);
+    }
+    expect: {
+        (function f(a = 0) {
+            console.log(function(b) {
+                a && b();
+                return a;
+            }(f));
+        })(42);
+    }
+    expect_stdout: [
+        "0",
+        "42",
+    ]
+    node_version: ">=6"
+}