Revert D15057633: [folly] allow conversion from nullptr to folly::Optional<bool>

Differential Revision:
D15057633

Original commit changeset: 6ba47a57a78e

fbshipit-source-id: 1ae1384e5e9de3f2e80571b626a41ef6e713b3b2

folly/Optional.h

@@ -133,6 +133,25 @@ class Optional {
     construct(newValue);
   }
 
+  /**
+   * Explicitly disallow converting nullptr to non-pointer
+   * types. Optional used to support initialization from nullptr as if
+   * it were folly::none. Without this constructor,
+   * folly::Optional<bool> could be constructed from nullptr,
+   * producing {false} instead of {}. This would be a change in
+   * behavior from the old code, so explicitly disallow it. Note that
+   * std::optional<bool> can be constructed from nullptr, also
+   * producing {false}.
+   *
+   * This constructor is temporary and should be removed when all call
+   * sites are fixed.
+   */
+  template <typename Null = std::nullptr_t>
+  /* implicit */
+  Optional(typename std::enable_if<
+           std::is_convertible<Null, Value>::value,
+           Null>::type) noexcept = delete;
+
   template <typename... Args>
   constexpr explicit Optional(in_place_t, Args&&... args) noexcept(
       std::is_nothrow_constructible<Value, Args...>::value)

folly/test/OptionalTest.cpp

@@ -667,18 +667,6 @@ TEST(Optional, InitializerListConstruct) {
   std::ignore = optional;
 }
 
-TEST(Optional, ConversionFromNullptr) {
-  // Match the behavior of std::optional
-  static constexpr void* null = nullptr;
-  folly::Optional<bool> o1{null};
-  folly::Optional<bool> o2(null);
-  folly::Optional<bool> o3 = null;
-
-  EXPECT_EQ(false, o1.value());
-  EXPECT_EQ(false, o2.value());
-  EXPECT_EQ(false, o3.value());
-}
-
 TEST(Optional, TestDisambiguationMakeOptionalVariants) {
   {
     auto optional = make_optional<int>(1);