The #line directive is added by the preprocessor and can then be used to help the developer understand which file and line a particular code fragment in the preprocessed file refers to. The #line directive tells code-processing tools to change the compiler’s internally stored line number and filename to a given line number and filename. Subsequent lines will be numbered relative to that position. Explicit preprocessing is mostly used for debugging or by various generators. In any case, a bug breaking this functionality may have a variety of negative effects. One of our users was faced with such a problem in Visual Studio 2019.