Is "sizeof new int;" UB?

code:

#include<iostream>

using namespace std;

int main() 
{
    size_t i = sizeof new int;

    cout<<i;
}

In GCC compiler, working fine without any warning or error and printed output 8.

But, in clang compiler, i got following warning:

warning: expression with side effects has no effect in an unevaluated context [-Wunevaluated-expression]
    size_t i = sizeof new int;

• Which one is true?
• Is sizeof new int; UB?