C++: lambda-expression in unevaluated context

Question

I am following this answer to define a priority_queue with a lambda function. However, I am running to: error: lambda-expression in unevaluated context

#include <bits/stdc++.h>

int main()
{
    std::priority_queue<
        int,
        std::vector<int>,
        decltype( [](int a, int b)->bool{
                   return a>b;
        })>
         q;
}

Answer 1

Your code is valid C++20 as written but invalid C++11.

• Lambda expressions are not allowed in unevaluated contexts (such as decltype) before C++20.
• Closure types are not default constructible before C++20. In C++20 a closure type that has no capture is default constructible.