Consider adding `__has_include`

[bonachea]

__has_include was added to CPP in C++17 and C23.

In a nutshell it provides a non-fatal probe of whether some proposed arguments for a given #include directive will succeed. The __has_include(file-name) expression evaluates to 1 if the search for the source file succeeds, and to ​0​ if the search fails. This enables programmatic fallback across potential header file names and more user-friendly error reporting for missing dependencies, all from within the preprocessor.

Here is a C++ example (modified from cppreference):

#if __has_include(<optional>)
#  include <optional>
#  define has_optional 1
   template<class T> using optional_t = std::optional<T>;
#elif __has_include(<experimental/optional>)
#  include <experimental/optional>
#  define has_optional -1
   template<class T> using optional_t = std::experimental::optional<T>;
#else
#  error No recognized header file found for the "optional" library. For more information on this error see ...
#endif

[gklimowicz]

This seems very useful. I need to look at existing codes to see if it is already in use. (I doubt it, but I would like to know for sure.)

Update 2025-02-02: Unsurprisingly, __has_include doesn't seem to appear in the existing Fortran code bases that I have collected.

[jeffhammond]

Fortran will not be like C/C++ in the usage of this. C/C++ have lots of system headers. Fortran has none. What Fortran headers will use this? In all of the projects I know that use include or #include, they are including files from the same source tree. There is no purpose to __has_include there except detecting in incomplete download or filesystem corruption.

I'm curious why we even want to encourage usage of include at this point. Isn't the goal to have stuff in modules?

[gklimowicz]

In the codes I have collected, more than 25% of the directive lines are #include. The main goal of the Fortran preprocessor effort is to standardize existing use, to support portability.

It's not a desire to discourage the use of modules, nor the upcoming generics features.

[jeffhammond]

It is vitally important to determine whether the uses of #include in those projects is consistent with the C usage of __has_include or not. Fortran headers from the same project are not going to use __has_include.