Make Optional<int> usable as constexpr (if initialized to none)

Summary: StorageTriviallyDestructible needed a constexpr ctor.

Reviewed By: yfeldblum, ot, luciang

Differential Revision: D14944492

fbshipit-source-id: cd9d39d7cb0719e7830887f548eabcc0defa1aee

folly/Optional.h

@@ -399,7 +399,8 @@ class Optional {
     };
     bool hasValue;
 
-    StorageTriviallyDestructible() : hasValue{false} {}
+    constexpr StorageTriviallyDestructible()
+        : emptyState('\0'), hasValue{false} {}
     void clear() {
       hasValue = false;
     }

folly/test/OptionalTest.cpp

@@ -63,6 +63,18 @@ static_assert(sizeof(Optional<short>) == sizeof(boost::optional<short>), "");
 static_assert(sizeof(Optional<int>) == sizeof(boost::optional<int>), "");
 static_assert(sizeof(Optional<double>) == sizeof(boost::optional<double>), "");
 
+TEST(Optional, ConstexprConstructible) {
+  // Use FOLLY_STORAGE_CONSTEXPR to work around MSVC not taking this.
+  static FOLLY_STORAGE_CONSTEXPR Optional<int> opt;
+  // NOTE: writing `opt = none` instead of `opt(none)` causes gcc to reject this
+  // code, claiming that the (non-constexpr) move ctor of `Optional` is being
+  // invoked.
+  static FOLLY_STORAGE_CONSTEXPR Optional<int> opt2(none);
+
+  EXPECT_FALSE(opt.has_value());
+  EXPECT_FALSE(opt2.has_value());
+}
+
 TEST(Optional, NoDefault) {
   Optional<NoDefault> x;
   EXPECT_FALSE(x);