Generalized support callables when std::invoke is available

Author: Morwenn

README.md

@@ -15,8 +15,9 @@ ones. `gfx::timsort` should be usable as a drop-in replacement for `std::stable_
 can't fallback to a O(n log² n) algorithm when there isn't enough extra heap memory available.
 
 Additionally `gfx::timsort` can take a [projection function](https://ezoeryou.github.io/blog/article/2019-01-22-ranges-projection.html)
-after the comparison function. The support is a bit rougher than in the linked article or the C++20 stadard library:
-only instances of types callable with parentheses can be used, there is no support for pointer to members.
+after the comparison function. The support is a bit rougher than in the linked article or the C++20 standard library:
+unless `std::invoke` is available, only instances of types callable with parentheses can be used, there is no support
+for pointer to members.
 
 This implementation of timsort notably avoids using the postfix `++` or `--` operators: only their prefix equivalents
 are used, which means that timsort will work even if the postfix operators are not present or return an incompatible

include/gfx/timsort.hpp

@@ -133,10 +133,17 @@ struct projection_compare {
 #if GFX_TIMSORT_USE_STD_MOVE
     template <typename T, typename U>
     bool operator()(T &&lhs, U &&rhs) {
+#   ifdef __cpp_lib_invoke
+        return static_cast<bool>(std::invoke(compare,
+            std::invoke(projection, std::forward<T>(lhs)),
+            std::invoke(projection, std::forward<U>(rhs))
+        ));
+#   else
         return static_cast<bool>(compare(
             projection(std::forward<T>(lhs)),
             projection(std::forward<U>(rhs))
         ));
+#   endif
     }
 #else
     template <typename T, typename U>