Using the below example file in a terminal emacs which isn't able to display the whole file without scrolling:

#include <cmath>

int main(void){

}

double other(void){
    ;
    ;
    ; // This will disappear
    ;
    ;
    ;
    ;
    ;
    ;
    ;
    ;
    ;
    ;
    ;
    ;
    ;
    ;
}

With C++-mode, flycheck-mode and company-mode (with the clang backend) activated i get the following behaviour when typing in acos into the main function. Note that the company completion starts at one character to highlight the behaviour, though the behaviour is the same with company-minimum-prefix-length set to 1: https://www.youtube.com/watch?v=Lg40mtSk_u8

I'm running:
GNU Emacs 24.3.1 (i686-pc-linux-gnu, GTK+ Version 3.10.7) of 2014-03-07 on toyol, modified by Debian
Flycheck version: 0.21alpha1 (package: 20141020.724)
Company version: 0.8.8-cvs