fix corner case in assignments (#5426)

fixes #5425

lib/compress.js

@@ -11139,9 +11139,15 @@ Compressor.prototype.compress = function(node) {
                 && assign instanceof AST_Assign
                 && assign.operator == "="
                 && self.left.equivalent_to(assign.left)) {
-                self.right = assign.right;
-                assign.right = self;
-                return assign;
+                return make_node(AST_Assign, self, {
+                    operator: "=",
+                    left: assign.left,
+                    right: make_node(AST_Binary, self, {
+                        operator: self.operator,
+                        left: self.left,
+                        right: assign.right,
+                    }),
+                }).optimize(compressor);
             }
         }
         if (compressor.option("comparisons")) switch (self.operator) {

test/compress/yields.js

@@ -1525,3 +1525,25 @@ issue_5385_2: {
     ]
     node_version: ">=10"
 }
+
+issue_5425: {
+    options = {
+        assignments: true,
+        ie: true,
+        toplevel: true,
+        unused: true,
+        yields: true,
+    }
+    input: {
+        var a = "FAIL";
+        var b = function* f() {}(a ? a = "PASS" : 42);
+        console.log(a, typeof f);
+    }
+    expect: {
+        var a = "FAIL";
+        (function* f() {})(a && (a = "PASS"));
+        console.log(a, typeof f);
+    }
+    expect_stdout: "PASS undefined"
+    node_version: ">=4"
+}