Clang:

<source>:3:21: warning: treating Unicode character <U+037E> as identifier character rather than as ';' symbol [-Wunicode-homoglyph]
    return num * num;

GCC lui est caca sur le coup:

<source>:3:18: warning: `num\\U0000037e' is not in NFC [-Wnormalized=]
    3 |     return num * num;
      |                  ^
<source>: In function 'int square(int)':
<source>:3:18: error: 'num\\U0000037e' was not declared in this scope; did you mean 'num'?
    3 |     return num * num;
      |                  ^~~~
      |                  num
<source>:3:23: error: expected ';' before '}' token
    3 |     return num * num;
      |                      ^
      |                      ;
    4 | }
      | ~