Yesterday I played with constexpr if a little, using clang++ built from the latest LLVM sources. C++17 will not have many of the features I was looking forward to, but this is a really nice convenience.
It lets us clean up some of those little templates-plus-specializations that only exist to execute different code in a parent template depending on the template’s type, particularly when the specialization is on a bool or integer value. It won’t help if the template specializations need different member variables or need to provide different type aliases or typedefs.
For instance, libsigc++-3.0 still had a little with_type template with partial template specializations for true and false. Replacing it with constexpr if makes the intention clearer.
I made many similar simplifications by using constexpr if in my murrayc-tuple-utils, which is used by libsigc++-3.0 via copy/paste/rename.
In my murrayc-dp-algorithms project, some code became much clearer and more efficient with constexpr if, allowing me to remove some template specializations that only existed to satisfy the compiler.