The following code is rejected by gcc:

#include <vector>

int main() {
   std::vector<decltype([] { 
  struct { int x; } x;
  return x; }()) > vec;
}
gcc claims that types cannot be defined inside of decltype. Hovewer, I did not find anything in standard to confirm such a message in this context. So if you need any proof, proof me please that there is a reason for gcc to claim that first.

https://godbolt.org/z/8n6djencY