fix corner case in collapse_vars (#5169)

fixes #5168

lib/compress.js

@@ -1501,7 +1501,7 @@ merge(Compressor.prototype, {
 
     AST_SymbolRef.DEFMETHOD("is_immutable", function() {
         var def = this.redef || this.definition();
-        return def.orig.length == 1 && def.orig[0] instanceof AST_SymbolLambda;
+        return (this.in_arg || def.orig.length == 1) && def.orig[0] instanceof AST_SymbolLambda;
     });
 
     AST_Node.DEFMETHOD("convert_symbol", noop);
@@ -3369,10 +3369,10 @@ merge(Compressor.prototype, {
                 }
             }
             statements.length = n;
-            CHANGED = n != len;
             if (has_quit) has_quit.forEach(function(stat) {
                 extract_declarations_from_unreachable_code(compressor, stat, statements);
             });
+            CHANGED = statements.length != len;
         }
 
         function sequencesize(statements, compressor) {

test/compress/destructured.js

@@ -3225,3 +3225,31 @@ issue_5153_object_var: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_5168: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        (function a({
+            [console.log(typeof function() {
+                ++a;
+                return a;
+            }())]: b,
+        }) {
+            var a;
+        })({});
+    }
+    expect: {
+        (function a({
+            [console.log(typeof function() {
+                ++a;
+                return a;
+            }())]: b,
+        }) {
+            var a;
+        })({});
+    }
+    expect_stdout: "function"
+    node_version: ">=6"
+}