Fix bug in folly::Optional::toStdOptional

Summary:
See https://fb.workplace.com/groups/145730152809376/permalink/1685600578822318/ for context. If you call toStdOptional on folly::Optional<bool>, the following cast will not work as expected:

```
folly::Optional<bool> optional = false;
// This will convert optional to bool and then construct std::optional<value>!
auto value = static_cast<std::optional<Value>>(optional);
```

So we just need to not rely on the cast and implement it in the more verbose way.

Differential Revision: D79928399

fbshipit-source-id: ea41ff6bf9c7724704b7d7056c0fd812e1faf0b6

Author: Yi Zhang

folly/Optional.h

@@ -220,11 +220,19 @@ class Optional {
   }
 
   std::optional<Value> toStdOptional() && noexcept {
-    return static_cast<std::optional<Value>>(std::move(*this));
+    if (has_value()) {
+      auto opt = std::optional<Value>(std::move(value()));
+      reset();
+      return opt;
+    }
+    return std::nullopt;
   }
 
   std::optional<Value> toStdOptional() const& noexcept {
-    return static_cast<std::optional<Value>>(*this);
+    if (has_value()) {
+      return std::optional<Value>(value());
+    }
+    return std::nullopt;
   }
 
   /// Set the Optional

folly/test/OptionalTest.cpp

@@ -844,6 +844,30 @@ TEST(Optional, StdOptionalConversions) {
   EXPECT_EQ(s, f.toStdOptional());
   EXPECT_TRUE(f);
 
+  folly::Optional<int> f_empty;
+  std::optional<int> s_empty = static_cast<std::optional<int>>(f_empty);
+  EXPECT_FALSE(s_empty.has_value());
+
+  folly::Optional<bool> bf = false;
+  std::optional<bool> bs = bf.toStdOptional();
+  EXPECT_EQ(bs.has_value(), true);
+  EXPECT_EQ(bs.value(), false);
+
+  folly::Optional<bool> bf_empty;
+  std::optional<bool> bs_empty = bf_empty.toStdOptional();
+  EXPECT_FALSE(bs_empty.has_value());
+
+  folly::Optional<bool> mbf = false;
+  auto mbs = std::move(mbf).toStdOptional();
+  EXPECT_TRUE(mbs.has_value());
+  EXPECT_FALSE(mbs.value());
+  EXPECT_FALSE(mbf.has_value());
+
+  folly::Optional<bool> mbf_empty;
+  auto mbs_empty = std::move(mbf_empty).toStdOptional();
+  EXPECT_FALSE(mbs_empty.has_value());
+  EXPECT_FALSE(mbf_empty.has_value());
+
   f = static_cast<folly::Optional<int>>(s);
   EXPECT_EQ(*f, 42);
   EXPECT_TRUE(s);