extern is a #202205171449.
By default, all defined functions and local variables are implicitly declared as extern. This means that the external sources can call the function defined in this file.
If a global variable is defined as extern, it will instead import the variable from where it is defined.
The expression could be variable or function (decay to 202202142131#). Therefore, depending on the type of X, it will find the most suitable expression for itself according to the result of controlling expression. The variables in controlling expression including the function parameters should not be qualified types (have identifier 202202111824#, 202202111830#, 202205171452#, 202205171504# and 202202081526#), array types and function types.