# 未实现错误：符号执行时不支持while_loop的梯度计算

- **ID:** `tensorflow/not-implemented-while-loop-gradients`
- **领域:** tensorflow
- **类别:** runtime_error
- **错误码:** `WLE`
- **验证级别:** ai_generated
- **修复率:** 80%

## 根因

带梯度的while循环需要自动图或显式梯度磁带；TF1图模式下的符号执行无法处理。

## 版本兼容性

| 版本 | 状态 | 引入 | 弃用 |
|------|------|------|------|
| tensorflow 2.12.0 | active | — | — |
| tensorflow 2.15.0 | active | — | — |

## 解决方案

1. ```
   Use tf.while_loop with back_prop=True inside a tf.function decorated with autograph: @tf.function; def train_step(): ...
   ```
2. ```
   Replace while_loop with vectorized operations using tf.map_fn or tf.scan.
   ```

## 无效尝试

- **** — Adding more GPU memory doesn't fix the algorithmic limitation. (90% 失败率)
- **** — Disabling eager mode makes symbolic execution worse. (80% 失败率)