Proof that https://twitter.com/MarcosBL/status/641110424193232897 will segfault in a program which passes “clang -Werror -Weverything -pedantic”

test.c

#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>
#include <float.h>

static int isNegative(float arg) {
    char *p = (char*) malloc(20);
    sprintf(p, "%f", arg);
    return p[0] == '-';
}

int main () {
    float f = FLT_MAX;
    printf("%f: %d", f, isNegative(f));
}