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Update flag forking requirement/recommendation to accomodate vendoring include/ #210

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33 changes: 25 additions & 8 deletions docs/beman_standard.md
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Expand Up @@ -833,18 +833,35 @@ entities in the `beman::<short_name>` namespace.
**Requirement**: C++ preprocessing must produce identical output regardless of compiler flags.

Therefore, feature test macros such as `__cpp_explicit_this_parameter` should
not be used directly. Instead use the following approach for feature-dependent
not be used directly.

**Recommendation**: Feature-dependent code should create `#define`s in generated headers based on CMake options, with a fallback for users who just vendor the include/ directory.

Instead use the following approach for feature-dependent
code generation:

1. Check for availability at CMake time using, for example,
`check_cxx_source_compiles`.
2. Create a CMake `option` (e.g. `BEMAN_<short_name>_USE_DEDUCING_THIS`)
with a default value based on detected support.
3. Generate a `config.hpp` with a `#define` macro set to the selected option.
4. Use this macro in place of the feature test macro.
1. Create a CMake `option` (e.g. `BEMAN_EXEMPLAR_USE_FEATURE_QUUX`).
2. Depending on the option, you may want to implement a check for its availability at
CMake time, using, for example, `check_cxx_source_compiles`, and set the option's
default value based on the result.
3. Generate a `config_generated.hpp` with a `#define` macro set to the selected option.
4. Create a `config.hpp` file that wraps `config_generated.hpp` with the following logic,
providing fallback defaults for users who vendor the include/ directory:

```cpp
#if !defined(__has_include) || \
__has_include(<beman/exemplar/config_generated.hpp>)
#include <beman/exemplar/config_generated.hpp>
#else
#define BEMAN_EXEMPLAR_USE_FEATURE_QUUX() 1
#endif
```

5. Include `config.hpp` in your headers, and use the resulting macro instead of the
feature test macro.

See
[beman.iterator_interface](https://github.com/bemanproject/iterator_interface/blob/5e6714e10faa1799723669e04abec6e75adbdb89/CMakeLists.txt#L44)
[beman.iterator_interface](https://github.com/bemanproject/iterator_interface/blob/473a9400fd51b9a4e3372a716ac1b501ec0297ad/CMakeLists.txt#L21)
for an example.

### **[cpp.extension_identifiers]**
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