Why is no warning given for the wrong use of __attribute__((pure)) in GCC?

I am trying to understand pure functions, and have been reading through the Wikipedia article on that topic. I wrote the minimal sample program as follows:

#include <stdio.h>

static int a = 1;

static __attribute__((pure)) int pure_function(int x, int y)
{
        return x + y;
}

static __attribute__((pure)) int impure_function(int x, int y)
{
        a++;
        return x + y;
}

int main(void)
{
        printf("pure_function(0, 0) = %d\n", pure_function(0, 0));
        printf("impure_function(0, 0) = %d\n", impure_function(0, 0));

        return 0;
}

I compiled this program with gcc -O2 -Wall -Wextra, expecting that an error, or at least a warning, should have been issued for decorating impure_function() with __attribute__((pure)). However, I received no warnings or errors, and the program also ran without issues.

Isn't marking impure_function() with __attribute__((pure)) incorrect? If so, why does it compile without any errors or warnings, even with the -Wextra and -Wall flags?

Thanks in advance!