I'd like to know when the size of a struct does not equal the combined size of it's members, in other words, when it is padded. gcc has a warning option to do exactly that, -Wpadded .
From https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html : "Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure."
Using this option didn't seem to catch padding where it should've though, so I wrote a simple test to demonstrate my issue:

#include <iostream>
#include <cstdint>

using namespace std;

struct test {
    uint16_t a;
    uint8_t b;
} test;

int main() {
    cout << sizeof(test) << ", " << alignof(test) << std::endl; 

    return 0;
}

compiling this with g++ test.cpp -Wpadded outputs warning: padding struct size to alignment boundary , and running the executable gives 4, 2 , exactly as I'd expect. Changing the struct to

struct test {
    uint8_t b;
    uint16_t a;
} test;

and running the resulting executable gives 4, 2 , but no warning is given.
Both structs have the same size and alignment, the size of uint8_t is obviously 1 byte, and uint16_t 2, so shouldn't I be getting a warning here? A page talking about various gcc options ( https://interrupt.memfault.com/blog/best-and-worst-gcc-clang-compiler-flags ) has a test similar to this one, and according to them, a warning is output in both situations.
I also ported the code to C (without the prints)

#include <stdint.h>

struct test {
    uint8_t b;
    uint16_t a;
} test;

int main() {
    return 0;
}

And got the same result.
I'm using gcc 8.1.0 from MinGW-W64.