How is "int main(){(([](){})());}" valid C++?

The code essentially calls an empty lambda.

Let's start from the beginning: [](){} is an empty lambda expression.

Then, in C and C++, you can wrap expressions in parens and they behave exactly the same^† as if written without them, so that's what the first pair of parens around the lambda does. We're now at ([](){}).

Then, () after the first wrapping parens calls the (empty) lambda. We're now at ([](){})()

The whole expression is wrapped in parens again and we get (([](){})()).

At last, ; ends the statement. We arrive at (([](){})());.

† There are some corner cases at least in C++, like with T a_var; there's a difference between decltype(a_var) and decltype((a_var)).