gcc rejects this valid syntax with

source>:2:32: error: expected ':' before '{' token
    2 |   auto b = requires { true ? [] {} : [] {}; };
      |                                ^~
      |                                :
<source>:2:33: error: expected primary-expression before '{' token
    2 |   auto b = requires { true ? [] {} : [] {}; };
      |                                 ^
<source>:2:35: error: expected ';' before ':' token
    2 |   auto b = requires { true ? [] {} : [] {}; };
      |                                   ^~
      |                                   ;

https://godbolt.org/z/4c3PPb5Pb

And gcc accepts the following invalid syntax:

int main() {
  auto b = requires { true ? [] : [] {}; };
}

https://godbolt.org/z/xeE1oYs1Y

(In reply to 康桓瑋 from comment #2)
> And gcc accepts the following invalid syntax:
> 
> int main() {
>   auto b = requires { true ? [] : [] {}; };
> }
> 
> https://godbolt.org/z/xeE1oYs1Y

It seems like the single [] makes gcc totally broken.

int main() {
  auto b = requires { true ? [] : []!-!-[]?[]; };
}

https://godbolt.org/z/EPTE6zf5E

maybe dup of PR99546.