declaration inside if condition

Assume it is valid C++ to include declaration inside the if condition (reference)

The following code segment compiles with a warning about the unused variable k (warning: unused variable ‘k’ [-Wunused-variable])

#include <iostream>
using std::cout;
using std::endl;

int test (void);

int main (void){

    //if((auto k = test()) != 0){
    if(auto k = test()){
        cout << "odd" << endl;
    }
    else{
        cout << "even" << endl;
    }

    return 0;
}

int test (void){
    static auto invocation_count = -1;
    ++invocation_count;
    return(invocation_count % 2);
}

But if the if condition is to be substituted by the line above (commented out), the following compilation errors ensue:

g++ -ggdb -std=c++17 -Wall -Werror=pedantic -Wextra  -c code.cpp
code.cpp: In function ‘int main()’:
code.cpp:9:9: error: expected primary-expression before ‘auto’
     if((auto k = test()) != 0){
         ^~~~
code.cpp:9:9: error: expected ‘)’ before ‘auto’
code.cpp:13:5: error: expected ‘)’ before ‘else’
     else{
     ^~~~
code.cpp:17:13: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]
     return 0;
             ^
make: *** [makefile:20: code.o] Error 1

Aren't the two if condition statements technically the same? Why is it that one compiles where as the other doesn't?