extend folly::split

Summary:
see task:
https://our.intern.facebook.com/intern/tasks/?t=4723861

Test Plan: unit tests

Reviewed By: marcelo.juchem@fb.com

FB internal diff: D1443223

Tasks: 4723861

folly/String.h

@@ -448,7 +448,8 @@ void splitTo(const Delim& delimiter,
 template <class T>
 using IsSplitTargetType = std::integral_constant<bool,
   std::is_arithmetic<T>::value ||
-  std::is_same<T, StringPiece>::value>;
+  std::is_same<T, StringPiece>::value ||
+  IsSomeString<T>::value>;
 
 template<bool exact = true,
          class Delim,

folly/test/StringTest.cpp

@@ -902,6 +902,49 @@ TEST(Split, fixed) {
   EXPECT_FALSE(folly::split('.', "a.b", a));
 }
 
+TEST(Split, std_string_fixed) {
+  std::string a, b, c, d;
+
+  EXPECT_TRUE(folly::split<false>('.', "a.b.c.d", a, b, c, d));
+  EXPECT_TRUE(folly::split<false>('.', "a.b.c", a, b, c));
+  EXPECT_TRUE(folly::split<false>('.', "a.b", a, b));
+  EXPECT_TRUE(folly::split<false>('.', "a", a));
+
+  EXPECT_TRUE(folly::split('.', "a.b.c.d", a, b, c, d));
+  EXPECT_TRUE(folly::split('.', "a.b.c", a, b, c));
+  EXPECT_TRUE(folly::split('.', "a.b", a, b));
+  EXPECT_TRUE(folly::split('.', "a", a));
+
+  EXPECT_TRUE(folly::split<false>('.', "a.b.c", a, b, c));
+  EXPECT_EQ("a", a);
+  EXPECT_EQ("b", b);
+  EXPECT_EQ("c", c);
+  EXPECT_FALSE(folly::split<false>('.', "a.b", a, b, c));
+  EXPECT_TRUE(folly::split<false>('.', "a.b.c", a, b));
+  EXPECT_EQ("a", a);
+  EXPECT_EQ("b.c", b);
+
+  EXPECT_TRUE(folly::split('.', "a.b.c", a, b, c));
+  EXPECT_EQ("a", a);
+  EXPECT_EQ("b", b);
+  EXPECT_EQ("c", c);
+  EXPECT_FALSE(folly::split('.', "a.b.c", a, b));
+  EXPECT_FALSE(folly::split('.', "a.b", a, b, c));
+
+  EXPECT_TRUE(folly::split<false>('.', "a.b", a, b));
+  EXPECT_EQ("a", a);
+  EXPECT_EQ("b", b);
+  EXPECT_FALSE(folly::split<false>('.', "a", a, b));
+  EXPECT_TRUE(folly::split<false>('.', "a.b", a));
+  EXPECT_EQ("a.b", a);
+
+  EXPECT_TRUE(folly::split('.', "a.b", a, b));
+  EXPECT_EQ("a", a);
+  EXPECT_EQ("b", b);
+  EXPECT_FALSE(folly::split('.', "a", a, b));
+  EXPECT_FALSE(folly::split('.', "a.b", a));
+}
+
 TEST(Split, fixed_convert) {
   StringPiece a, d;
   int b;