Support std::initializer_list in folly::range

Summary:
Support `std::initializer_list<T>` in the `folly::range` and `folly::crange` helper functions, returning `folly::Range<const T*>`.

This is to make it easier to forward `std::initializer_list<T>` parameters to `folly::Range` ones in customer code.

Reviewed By: ot

Differential Revision: D54115340

fbshipit-source-id: d47b2fee73415890f746dba578f0d7770c90637e

folly/Range.h

@@ -1298,6 +1298,16 @@ constexpr Range<T const*> crange(std::array<T, n> const& array) {
   return Range<T const*>{array};
 }
 
+template <class T>
+constexpr Range<T const*> range(std::initializer_list<T> ilist) {
+  return Range<T const*>(ilist.begin(), ilist.end());
+}
+
+template <class T>
+constexpr Range<T const*> crange(std::initializer_list<T> ilist) {
+  return Range<T const*>(ilist.begin(), ilist.end());
+}
+
 using StringPiece = Range<const char*>;
 using MutableStringPiece = Range<char*>;
 using ByteRange = Range<const unsigned char*>;

folly/test/RangeTest.cpp

@@ -1631,6 +1631,22 @@ TEST(Range, MutableStringPieceExplicitConversionOperator) {
   EXPECT_EQ("hello", piecec.to<fake_string_view>(fake_tag{}));
 }
 
+TEST(Range, InitializerList) {
+  auto check = [](Range<int const*> r) {
+    ASSERT_EQ(r.size(), 3);
+    EXPECT_EQ(*r.begin(), 1);
+    EXPECT_EQ(*(r.begin() + 1), 2);
+    EXPECT_EQ(*(r.begin() + 2), 3);
+  };
+
+  check(range({1, 2, 3}));
+  check(crange({1, 2, 3}));
+
+  static constexpr auto ilist = {1, 2, 3};
+  check(range(ilist));
+  check(crange(ilist));
+}
+
 #if FOLLY_HAS_STRING_VIEW
 namespace {
 std::size_t stringViewSize(std::string_view s) {