Fix tryTo to support conversion to enumerations

Summary:
[Folly] Fix `tryTo` to support conversion to enumerations.

`tryTo` should return `Expected<Tgt, ConversionCode>` instead of `Tgt`.

Reviewed By: yfeldblum

Differential Revision: D5144706

fbshipit-source-id: cd23f3cf75de7c5a26bc569f3cb47fff360f6e2a

folly/Conv.h

@@ -1536,7 +1536,7 @@ tryTo(const Src& value) {
 template <class Tgt, class Src>
 typename std::enable_if<
     std::is_enum<Tgt>::value && !std::is_same<Src, Tgt>::value,
-    Tgt>::type
+    Expected<Tgt, ConversionCode>>::type
 tryTo(const Src& value) {
   using I = typename std::underlying_type<Tgt>::type;
   return tryTo<I>(value).then([](I i) { return static_cast<Tgt>(i); });

folly/test/ConvTest.cpp

@@ -1061,6 +1061,18 @@ TEST(Conv, TryStringToInt) {
   EXPECT_EQ(rv2.value(), 4711);
 }
 
+TEST(Conv, TryStringToEnum) {
+  enum class A { x = 42, y = 420, z = 65 };
+  auto rv1 = folly::tryTo<A>("1000000000000000000000000000000");
+  EXPECT_FALSE(rv1.hasValue());
+  auto rv2 = folly::tryTo<A>("42");
+  EXPECT_TRUE(rv2.hasValue());
+  EXPECT_EQ(A::x, rv2.value());
+  auto rv3 = folly::tryTo<A>("50");
+  EXPECT_TRUE(rv3.hasValue());
+  EXPECT_EQ(static_cast<A>(50), rv3.value());
+}
+
 TEST(Conv, TryStringToFloat) {
   auto rv1 = folly::tryTo<float>("");
   EXPECT_FALSE(rv1.hasValue());