From 9d1cf196cf59cef8ac8da8ea95d896965f8bc22d Mon Sep 17 00:00:00 2001
From: "Pavel I. Kryukov" <kryukov@frtk.ru>
Date: Tue, 8 Mar 2022 11:38:19 +0300
Subject: [PATCH] Make enum_fuse typesafe (fixes #143) (#145)

---
 doc/reference.md       | 11 +++++++++--
 include/magic_enum.hpp | 16 ++++++++++++++--
 test/test.cpp          | 14 +++++++++++---
 3 files changed, 34 insertions(+), 7 deletions(-)

diff --git a/doc/reference.md b/doc/reference.md
index 9266752..adbf075 100644
--- a/doc/reference.md
+++ b/doc/reference.md
@@ -330,10 +330,16 @@ constexpr string_view enum_type_name() noexcept;
 
 ```cpp
 template <typename... Es>
-[[nodiscard]] constexpr optional<std::uintmax_t> enum_fuse(Es... values);
+[[nodiscard]] constexpr optional<FusedEnum> enum_fuse(Es... values);
 ```
 
-* Returns a bijective mix of several enum values. This can be used to emulate 2D switch/case statements.
+* Returns a typesafe bijective mix of several enum values. This can be used to emulate 2D switch/case statements.
+
+* Return type is `optional<FusedEnum>` where FusedEnum is an incomplete enum. It is unique for any given combination of `Es...`.
+
+* Switch/case statement over an incomplete enum is a Visual Studio warning C4064
+  * You have to silent (/wd4064) or ignore it.
+  * Alternatively, define MAGIC_ENUM_NO_TYPESAFE_ENUM_FUSE to disable type-safety (FusedEnum equals std::uintmax_t).
 
 * Examples
 
@@ -341,6 +347,7 @@ template <typename... Es>
   switch (magic_enum::enum_fuse(color, direction).value()) {
     case magic_enum::enum_fuse(Color::RED, Directions::Up).value(): // ...
     case magic_enum::enum_fuse(Color::BLUE, Directions::Down).value(): // ...
+    case magic_enum::enum_fuse(Directions::BLUE, Color::Down).value(): // Compilation error
   // ...
   }
   ```
diff --git a/include/magic_enum.hpp b/include/magic_enum.hpp
index 628cfc0..ea5339f 100644
--- a/include/magic_enum.hpp
+++ b/include/magic_enum.hpp
@@ -917,8 +917,7 @@ template <typename E>
 constexpr optional<std::uintmax_t> fuse_one_enum(optional<std::uintmax_t> hash, E value) noexcept {
   if (hash.has_value()) {
     if (const auto index = enum_index(value); index.has_value()) {
-      // Add 1 to prevent matching 2D fusions with 3D fusions etc.
-      return (hash.value() << detail::log2(enum_count<E>() + 1)) | (index.value() + 1);
+      return (hash.value() << detail::log2(enum_count<E>() + 1)) | index.value();
     }
   }
   return {};
@@ -934,6 +933,15 @@ constexpr optional<std::uintmax_t> fuse_enum(E head, Es... tail) noexcept {
   return fuse_one_enum(fuse_enum(tail...), head);
 }
 
+template <typename... Es>
+constexpr auto typesafe_fuse_enum(Es... values) noexcept {
+  enum class Enum : std::uintmax_t;
+  auto hash = fuse_enum(values...);
+  if (hash.has_value())
+    return optional(static_cast<Enum>(hash.value()));
+  return optional<Enum>{};
+}
+
 } // namespace magic_enum::fusion_detail
 
 // Returns a bijective mix of several enum values. This can be used to emulate 2D switch/case statements.
@@ -942,7 +950,11 @@ template <typename... Es>
   static_assert((std::is_enum_v<std::decay_t<Es>> && ...), "magic_enum::enum_fuse works only with enums");
   static_assert(sizeof...(Es) >= 2, "magic_enum::enum_fuse requires at least 2 enums");
   static_assert((detail::log2(enum_count<Es>() + 1) + ...) <= (sizeof(std::uintmax_t) * 8), "magic_enum::enum_fuse does not work for large enums");
+#ifdef MAGIC_ENUM_NO_TYPESAFE_ENUM_FUSE
   const auto fuse = fusion_detail::fuse_enum(values...);
+#else
+  const auto fuse = fusion_detail::typesafe_fuse_enum(values...);
+#endif
   return assert(fuse.has_value()), fuse;
 }
 
diff --git a/test/test.cpp b/test/test.cpp
index 3ffdd12..84fea6e 100644
--- a/test/test.cpp
+++ b/test/test.cpp
@@ -1026,6 +1026,11 @@ TEST_CASE("constexpr_for") {
   });
 }
 
+#ifdef _MSC_VER
+# pragma warning(push)
+# pragma warning(disable : 4064)
+#endif
+
 static int switch_case_2d(Color color, Directions direction) {
   switch (magic_enum::enum_fuse(color, direction).value()) {
     case magic_enum::enum_fuse(Color::RED, Directions::Up).value():
@@ -1043,21 +1048,24 @@ static int switch_case_3d(Color color, Directions direction, Index index) {
   switch (magic_enum::enum_fuse(color, direction, index).value()) {
     case magic_enum::enum_fuse(Color::RED, Directions::Up, Index::zero).value():
       return 1;
-    // model accidental removal of last index, must not match anything
-    case magic_enum::enum_fuse(Color::BLUE, Directions::Up).value():
+    case magic_enum::enum_fuse(Color::BLUE, Directions::Up, Index::zero).value():
       return 2;
     default:
       return 0;
   }
 }
 
+#ifdef _MSC_VER
+#  pragma warning(pop)
+#endif
+
 TEST_CASE("multdimensional-switch-case") {
   REQUIRE(switch_case_2d(Color::RED, Directions::Up) == 1);
   REQUIRE(switch_case_2d(Color::RED, Directions::Down) == 0);
   REQUIRE(switch_case_2d(Color::BLUE, Directions::Up) == 0);
   REQUIRE(switch_case_2d(Color::BLUE, Directions::Down) == 2);
   REQUIRE(switch_case_3d(Color::RED, Directions::Up, Index::zero) == 1);
-  REQUIRE(switch_case_3d(Color::BLUE, Directions::Up, Index::zero) == 0);
+  REQUIRE(switch_case_3d(Color::BLUE, Directions::Up, Index::zero) == 2);
   REQUIRE(switch_case_3d(Color::BLUE, Directions::Up, Index::one) == 0);
   REQUIRE(switch_case_3d(Color::BLUE, Directions::Up, Index::two) == 0);
 }