How to make a for loop variable const with the exception of the increment statement?

From c++20, you can use ranges::views::iota like this:

for (int const i : std::views::iota(0, 10))
{
   std::cout << i << " ";  // ok
   i = 42;                 // error
}

Here’s a demo.

From c++11, you can also use the following technique, which uses an IIILE (immediately invoked inline lambda expression):

int x = 0;
for (int i = 0; i < 10; ++i) [&,i] {
    std::cout << i << " ";  // ok, i is readable
    i = 42;                 // error, i is captured by non-mutable copy
    x++;                    // ok, x is captured by mutable reference
}();     // IIILE

Here’s a demo.

Note that [&,i] means that i is captured by non-mutable copy, and everything else is captured by mutable reference. The (); at the end of the loop simply means that the lambda is invoked immediately.