error: expected primary-expression before ‘auto’

For reference: C++17 std::optional error: expected primary-expression before 'auto'

My test code is below. My understanding is that my usage of auto in this context is as outlined in the above reference for valid C++ code. However, I still get the error:

error: expected primary-expression before ‘auto’
   if((auto pos = s2.find(c)) != -1)

What am I missing?

#include<string>
#include<set>
#include<algorithm>
#include<iostream>

using std::string;
using std::set;
using std::for_each;
using std::cout;
using std::endl;

void compare(const string& s1, const string& s2)
{
  set<size_t>sequence;

  for_each(s1.begin(),s1.end(),[&](const char& c){

          if((auto pos = s2.find(c)) != -1)
          {
            sequence.insert(pos);
          }
       }
      );

  return;
}

int func() 
{
  string s1 = "some string";
  string s2 = "some other string";

  compare(s1,s2);

  return 0; 
}

int main() {
  auto exit = (int (*)()) &func;

  std::cout << exit() << std::endl;
}

The format for intializing a variable in an if statement and then using said variable in a condition is:

if(type name = initiallizer; condition)

So, for this case you need:

if(auto pos = s2.find(c); pos != -1)

Although you should use std::string::npos instead of -1 to avoid magic numbers in your code.