fix corner case in ie8 (#5040)

fixes #5039

lib/compress.js

@@ -7861,11 +7861,13 @@ merge(Compressor.prototype, {
             var prop = this.property;
             if (expr.may_throw_on_access(compressor)) {
                 if (!this.optional) return this;
-                prop = prop.drop_side_effect_free(compressor);
-                if (!prop) return expr.drop_side_effect_free(compressor, first_in_statement);
-                var node = this.clone();
-                node.property = prop;
-                return node;
+                if (prop.has_side_effects(compressor)) {
+                    prop = prop.drop_side_effect_free(compressor);
+                    if (!prop) return expr.drop_side_effect_free(compressor, first_in_statement);
+                    var node = this.clone();
+                    node.property = prop;
+                    return node;
+                }
             }
             expr = expr.drop_side_effect_free(compressor, first_in_statement);
             if (!expr) return prop.drop_side_effect_free(compressor, first_in_statement);

test/compress/optional-chains.js

@@ -305,3 +305,25 @@ issue_4947_2: {
     expect_stdout: "PASS"
     node_version: ">=14"
 }
+
+issue_5039: {
+    options = {
+        ie8: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = a?.[function f() {
+            f;
+            a;
+        }];
+        console.log("PASS");
+    }
+    expect: {
+        (function f() {});
+        console.log("PASS");
+    }
+    expect_stdout: "PASS"
+    node_version: ">=14"
+}