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I wasn't sure if the correct component was 'event' or 'video', since the commit that broke this behaviour affects both.

I use SDL with X11 under Linux (Debian Wheezy with some packages from Jessie, Stretch and Experimental). When I compile and run this bare bones program:

#include <stdio.h>
#include "SDL.h"
int main()
{
    SDL_bool done = SDL_FALSE;
    SDL_Init(0);
    SDL_CreateWindow("Window Title", 0, 0, 640, 480, SDL_WINDOW_SHOWN);
    while (!done) {
        SDL_Event event;
        if (SDL_PollEvent(&event)) {
            switch (event.type) {
                case SDL_QUIT:
                    done = SDL_TRUE;
                    break;
                case SDL_KEYDOWN:
                    printf("0x%X", event.key.keysym.sym);
                    printf(" %d\n", event.key.keysym.sym == SDLK_RALT);
            }
        }
    }
    SDL_Quit();
    return 0;
}

and press AltGr, with SDL 2.0.3 it displays:

0x400000E6 1

which means that the keysym is SDLK_RALT. This is OK. However, when I run it with SDL 2.0.4, it displays:

0x40000000 0

Given that 0x40000000 is not associated to any SDLK_xxx constant, I believe that this is a bug.

By bisecting I've found that the problem was introduced in this commit: http://hg.libsdl.org/SDL/rev/9e8323b058d6

The other keys seem to work fine. In particular, RSHIFT, RCTRL, LSHIFT, LCTRL, LALT all generate the expected keysym.sym value.

In case it's important, my keyboard layout is Spanish, and to my recollection (which isn't that good) I have only remapped Scroll Lock and no other key.