Prior to C99 (i.e., in C90), there was a rule that any undeclared identifier id that was followed by a function call parenthesis would be treated as though there were an implicit declaration in the scope in which it was used like this:

  extern int id();

The empty parentheses meant it could take any number of arguments, since it was declared in the traditional style, where the argument types weren't specified first.

This implicit declaration meant that if it was later defined in the same file as returning a type other than int, the compiler wasn't permitted to go back up in the file and treat the function call as though it returned a type other than int.

This requirement was removed in C99, but in practice, compilers kept doing it for backwards compatibility, even when not invoked in C90 mode.