为什么 Generator expression must be parenthesized？

[JasonLaw]

我在做First Unique Character in a String - LeetCode的时候，碰到“SyntaxError: Generator expression must be parenthesized”。

我想知道的是，多个参数时，为什么 Generator expression must be parenthesized ？

# 重现代码
class Solution:
    def firstUniqChar(self, s: str) -> int:
        char_to_index = {}
        for i, c in enumerate(s):
            if c in char_to_index:
                char_to_index[c] = -1
            else:
                char_to_index[c] = i

        return min(
            index for index in char_to_index.values() if index != -1,
            default=-1
        )

[jfcherng]

https://github.com/python/cpython/pull/4382