On-heap struct pointer assignment corrupts memory

[ChAoSUnItY]

Considering the following example minimum arena implementation:

#include <stdlib.h>

typedef enum {
    A,
    B
} type_t;

typedef struct {
    type_t typ;
} data_t;

typedef struct {
    int cap;
    int size;
    data_t *data;
} arena_t;

arena_t *arena_init(int cap) {
    arena_t *arena = malloc(sizeof(arena_t));
    arena->cap = cap;
    arena->size = 0;
    arena->data = calloc(cap, sizeof(data_t));
    return arena;
}

data_t *arena_alloc(arena_t *arena) {
    if (arena->size + 1 >= arena->cap)
        return NULL;

    return &arena->data[arena->size++];
}

void arena_free(arena_t *arena) {
    free(arena->data);
    free(arena);
}

int main() {
    arena_t *arena = arena_init(1024);
    data_t *data = arena_alloc(arena);
    printf("%d\n", arena->data);
    data->typ = B;
    printf("%d\n", arena->data);
    arena_free(arena);

    return 0;
}

I expect that it would output 2 numbers that refers to same memory address where arena->data resides, however, using shecc results in the second number is different to the first one, and same as the integer value of enum B previously assigned.

Using gcc to compile this program would have expected result however.

[ChAoSUnItY]

Additional note, this seems to not be only affecting by enum, in fact, all types which resides in the first member of struct would cause same behavior.

This doesn't affect if the array is allocated outside of a struct on heap, the following code works as expected:

#include <stdlib.h>

typedef enum {
    A,
    B
} type_t;

typedef struct {
    type_t typ;
} data_t;


int cap = 1024;
int size = 0;
data_t *data;

int main() {
    data = calloc(cap, sizeof(data_t));
    data_t *local_data = &data[size++];
    printf("%d\n", data);
    local_data->typ = A;
    printf("%d\n", data);
    free(data);

    return 0;
}