feat(utility): add bind_front for enum_switch partial application (#200)

doc/limitations.md

@@ -21,6 +21,8 @@
 
 * Zero is not reflected for flag enums.
 
+* GCC 16 with `-std=c++26` may miscompile string parsing that uses `find`/`remove_prefix` on flag names. `enum_flags_cast` uses an explicit token loop instead; see [#467](https://github.com/Neargye/magic_enum/issues/467).
+
 ## Enum Range
 
 * For non-flag enums, range-based reflection only considers values in `[MAGIC_ENUM_RANGE_MIN, MAGIC_ENUM_RANGE_MAX]`.

doc/reference.md

@@ -511,6 +511,28 @@ constexpr auto enum_for_each(Lambda&& lambda);
   });
   ```
 
+## `bind_front`
+
+```cpp
+template <typename F, typename... Args>
+constexpr auto bind_front(F&& f, Args&&... args);
+```
+
+* Defined in header `<magic_enum/magic_enum_utility.hpp>`
+
+* C++17-compatible partial application helper. Useful with `enum_switch` to avoid nested lambdas on MSVC ([#200](https://github.com/Neargye/magic_enum/issues/200)).
+
+* Example
+
+  ```cpp
+  enum class Shape { Circle, Square };
+  auto describe = [](Shape s, std::string_view prefix) {
+    return std::string(prefix) + magic_enum::enum_name(s).data();
+  };
+  auto describe_circle = magic_enum::bind_front(describe, Shape::Circle);
+  describe_circle("shape: "); // -> "shape: Circle"
+  ```
+
 ## `enum_next_value` and `enum_prev_value`
 
 ```cpp

include/magic_enum/magic_enum_utility.hpp

@@ -36,6 +36,7 @@
 
 #ifndef MAGIC_ENUM_USE_STD_MODULE
 #  include <tuple>
+#  include <utility>
 #endif
 
 namespace magic_enum {
@@ -144,6 +145,32 @@ template <typename E, detail::enum_subtype S = detail::subtype_v<E>>
   return MAGIC_ENUM_ASSERT(false), value;
 }
 
+template <typename F, typename... BoundArgs>
+class bind_front_t {
+ public:
+  template <typename Fn, typename... Args>
+  constexpr explicit bind_front_t(Fn&& fn, Args&&... args)
+      : fn_(std::forward<Fn>(fn)), bound_(std::forward<Args>(args)...) {}
+
+  template <typename... CallArgs>
+  constexpr decltype(auto) operator()(CallArgs&&... call_args) const {
+    return std::apply(
+        fn_,
+        std::tuple_cat(bound_,
+                       std::forward_as_tuple(std::forward<CallArgs>(call_args)...)));
+  }
+
+ private:
+  F fn_;
+  std::tuple<BoundArgs...> bound_;
+};
+
+template <typename F, typename... Args>
+[[nodiscard]] constexpr auto bind_front(F&& f, Args&&... args) {
+  return bind_front_t<std::decay_t<F>, std::decay_t<Args>...>{
+      std::forward<F>(f), std::forward<Args>(args)...};
+}
+
 } // namespace magic_enum
 
 #endif // NEARGYE_MAGIC_ENUM_UTILITY_HPP

test/test.cpp

@@ -1315,6 +1315,19 @@ TEST_CASE("enum_for_each") {
   }
 }
 
+TEST_CASE("bind_front") {
+  auto add = [](int a, int b, int c) { return a + b + c; };
+  auto add_ab = magic_enum::bind_front(add, 1, 2);
+  REQUIRE(add_ab(3) == 6);
+
+  enum class Shape { Circle, Square };
+  auto describe = [](Shape s, std::string_view prefix) {
+    return std::string(prefix) + magic_enum::enum_name(s).data();
+  };
+  auto describe_circle = magic_enum::bind_front(describe, Shape::Circle);
+  REQUIRE(describe_circle("shape: ") == "shape: Circle");
+}
+
 #if defined(__clang__) && __clang_major__ >= 5 || defined(__GNUC__) && __GNUC__ >= 9 || defined(_MSC_VER) && _MSC_VER >= 1920
 #  define MAGIC_ENUM_SUPPORTED_CONSTEXPR_FOR 1
 #endif