fix corner case in unused (#4925)

fixes #4924

lib/compress.js

@@ -7415,9 +7415,7 @@ merge(Compressor.prototype, {
             }
             if (left.has_side_effects(compressor)) return this;
             var right = this.right;
-            if (lazy_op[this.operator.slice(0, -1)]) {
-                this.write_only = !right.has_side_effects(compressor);
-            } else {
+            if (!lazy_op[this.operator.slice(0, -1)]) {
                 this.write_only = true;
                 if (root_expr(left).is_constant_expression(compressor.find_parent(AST_Scope))) {
                     return right.drop_side_effect_free(compressor);

test/compress/assignments.js

@@ -701,3 +701,48 @@ issue_4876: {
     expect_stdout: "PASS"
     node_version: ">=15"
 }
+
+issue_4924_1: {
+    options = {
+        collapse_vars: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a, b;
+        console.log("PASS");
+        a = function() {};
+        b = function() {}(b ||= a);
+    }
+    expect: {
+        var b;
+        console.log("PASS");
+        b = void (b ||= function() {});
+    }
+    expect_stdout: "PASS"
+    node_version: ">=15"
+}
+
+issue_4924_2: {
+    options = {
+        collapse_vars: true,
+        dead_code: true,
+        passes: 2,
+        sequences: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a, b;
+        console.log("PASS");
+        a = function() {};
+        b = function() {}(b ||= a);
+    }
+    expect: {
+        console.log("PASS");
+    }
+    expect_stdout: "PASS"
+    node_version: ">=15"
+}