Why does GCC only complain about an unsigned int argument being used with %i when -pedantic is given?

Question

sscanf(3) says (emphasis mine):

i
Matches an optionally signed integer; the next pointer must be a pointer to int. The integer is read in base 16 if it begins with 0x or 0X, in base 8 if it begins with 0, and in base 10 otherwise. Only characters that correspond to the base are used.

However GCC does not complain about using an unsigned int with %i unless -pedantic is given. This is different from the behavior I'm used to, where GCC will warn for any mismatched type and format string.

Why does this combination behave differently?

Given that this warning is not included in the common -Wall set of warnings, is it acceptable to pass unsigned int to %i?

---

Example program:

#include <stdio.h>

int main(void)
{
    int i;
    unsigned int u;
    float f;

    scanf("%i", &i);
    scanf("%i", &u);
    scanf("%i", &f);

    return 0;
}

Without -pedantic, GCC complains about %i and float *, but not unsigned int *:

$ gcc -Wall -Wextra scanf_i.c 
scanf_i.c: In function ‘main’:
scanf_i.c:11:13: warning: format ‘%i’ expects argument of type ‘int *’, but argument 2 has type ‘float *’ [-Wformat=]
     scanf("%i", &f);
            ~^   ~~
            %e

With -pedantic, GCC complains about both: Output with -pedantic:

$ gcc -Wall -Wextra -pedantic scanf_i.c 
scanf_i.c: In function ‘main’:
scanf_i.c:10:13: warning: format ‘%i’ expects argument of type ‘int *’, but argument 2 has type ‘unsigned int *’ [-Wformat=]
     scanf("%i", &u);
            ~^   ~~
            %i
scanf_i.c:11:13: warning: format ‘%i’ expects argument of type ‘int *’, but argument 2 has type ‘float *’ [-Wformat=]
     scanf("%i", &f);
            ~^   ~~
            %e

GCC version:

$ gcc --version
gcc (Debian 8.3.0-6) 8.3.0

Answer 1

-Wformat-signedness controls whether warnings are raised when the argument type differs only in signedness from what's expected.

From the gcc(1) man page:

-Wformat-signedness
If -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa.

The man page does not explicitly state that this flag is included in -pedantic, (-Wpedantic) but it does say:

However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version.