From a5683e130c2fc7ca2e20628414cc3eb490c52aea Mon Sep 17 00:00:00 2001
From: Michael Park <mcypark@fb.com>
Date: Sat, 22 Aug 2020 01:42:46 -0700
Subject: [PATCH] Add `std::variant` support for `folly::variant_match`.

Summary: This patch adds support for `std::variant` to `folly::variant_match` but without including `<variant>`.

Reviewed By: yfeldblum

Differential Revision: D23196572

fbshipit-source-id: 9d5b519958b04c13cc024d393247c73cec00ac67
---
 folly/Overload.h            | 23 +++++++++++++-
 folly/test/OverloadTest.cpp | 61 +++++++++++++++++++++++++++++++++----
 2 files changed, 77 insertions(+), 7 deletions(-)

diff --git a/folly/Overload.h b/folly/Overload.h
index 5b3e0b2dd2a..4e6e8ae19e0 100644
--- a/folly/Overload.h
+++ b/folly/Overload.h
@@ -19,6 +19,9 @@
 #include <type_traits>
 #include <utility>
 
+#include <folly/Traits.h>
+#include <folly/functional/Invoke.h>
+
 /**
  * folly implementation of `std::overload` like functionality
  *
@@ -64,15 +67,33 @@ decltype(auto) overload(Cases&&... cases) {
       std::forward<Cases>(cases)...};
 }
 
+namespace overload_detail {
+FOLLY_CREATE_MEMBER_INVOKER(valueless_by_exception, valueless_by_exception);
+FOLLY_CREATE_FREE_INVOKER(visit, visit);
+FOLLY_CREATE_FREE_INVOKER(apply_visitor, apply_visitor);
+} // namespace overload_detail
+
 /*
  * Match `Variant` with one of the `Cases`
  *
  * Note: you can also use `[] (const auto&) {...}` as default case
  *
+ * Selects `visit` if `v.valueless_by_exception()` available and the call to
+ * `visit` is valid (e.g. `std::variant`). Otherwise, selects `apply_visitor`
+ * (e.g. `boost::variant`, `folly::DiscriminatedPtr`).
  */
 template <typename Variant, typename... Cases>
 decltype(auto) variant_match(Variant&& variant, Cases&&... cases) {
-  return apply_visitor(
+  using invoker = std::conditional_t<
+      folly::Conjunction<
+          is_invocable<overload_detail::valueless_by_exception, Variant>,
+          is_invocable<
+              overload_detail::visit,
+              decltype(overload(std::forward<Cases>(cases)...)),
+              Variant>>::value,
+      overload_detail::visit,
+      overload_detail::apply_visitor>;
+  return invoker{}(
       overload(std::forward<Cases>(cases)...), std::forward<Variant>(variant));
 }
 
diff --git a/folly/test/OverloadTest.cpp b/folly/test/OverloadTest.cpp
index 11169f5078e..0b79ab40af8 100644
--- a/folly/test/OverloadTest.cpp
+++ b/folly/test/OverloadTest.cpp
@@ -15,6 +15,9 @@
  */
 
 #include <folly/Overload.h>
+
+#include <variant>
+
 #include <boost/variant.hpp>
 #include <folly/DiscriminatedPtr.h>
 #include <folly/portability/GTest.h>
@@ -32,11 +35,29 @@ struct Two {
     return "Two";
   }
 };
-using OneOrTwo = boost::variant<One, Two>;
+
+TEST(Overload, StdVariant) {
+  using V = std::variant<One, Two>;
+  V one(One{});
+  V two(Two{});
+
+  EXPECT_TRUE(variant_match(
+      one, [](const One&) { return true; }, [](const Two&) { return false; }));
+  EXPECT_TRUE(variant_match(
+      two, [](const One&) { return false; }, [](const Two&) { return true; }));
+
+  auto toString = [](const auto& variant) {
+    return variant_match(
+        variant, [](const auto& value) { return value.toString(); });
+  };
+  EXPECT_EQ(toString(one), "One");
+  EXPECT_EQ(toString(two), "Two");
+}
 
 TEST(Overload, BoostVariant) {
-  OneOrTwo one(One{});
-  OneOrTwo two(Two{});
+  using V = boost::variant<One, Two>;
+  V one(One{});
+  V two(Two{});
 
   EXPECT_TRUE(variant_match(
       one, [](const One&) { return true; }, [](const Two&) { return false; }));
@@ -75,9 +96,36 @@ TEST(Overload, DiscriminatedPtr) {
   EXPECT_EQ(toString(two_ptr), "Two");
 }
 
-TEST(Overload, Pattern) {
-  OneOrTwo one(One{});
-  OneOrTwo two(Two{});
+TEST(Overload, StdPattern) {
+  using V = std::variant<One, Two>;
+  V one(One{});
+  V two(Two{});
+
+  auto is_one_overload = overload(
+      [](const One&) { return true; }, [](const Two&) { return false; });
+  EXPECT_TRUE(std::visit(is_one_overload, one));
+  EXPECT_TRUE(variant_match(one, is_one_overload));
+  EXPECT_FALSE(variant_match(two, is_one_overload));
+
+  auto is_two_overload = overload(
+      [](const One&) { return false; }, [](const Two&) { return true; });
+  EXPECT_TRUE(std::visit(is_two_overload, two));
+  EXPECT_FALSE(variant_match(one, is_two_overload));
+  EXPECT_TRUE(variant_match(two, is_two_overload));
+
+  auto is_one_copy = overload(is_one_overload);
+  auto is_one_const_copy =
+      overload(static_cast<const decltype(is_one_overload)&>(is_one_overload));
+  EXPECT_TRUE(variant_match(one, is_one_copy));
+  EXPECT_TRUE(variant_match(one, is_one_const_copy));
+  EXPECT_FALSE(variant_match(two, is_one_copy));
+  EXPECT_FALSE(variant_match(two, is_one_const_copy));
+}
+
+TEST(Overload, BoostPattern) {
+  using V = boost::variant<One, Two>;
+  V one(One{});
+  V two(Two{});
 
   auto is_one_overload = overload(
       [](const One&) { return true; }, [](const Two&) { return false; });
@@ -99,5 +147,6 @@ TEST(Overload, Pattern) {
   EXPECT_FALSE(variant_match(two, is_one_copy));
   EXPECT_FALSE(variant_match(two, is_one_const_copy));
 }
+
 } // namespace test
 } // namespace folly