fbstring::append, fbstring::operator+= overloads for string_view

Summary: Since C++17, `std::basic_string::append` and `std::basic_string::operator+=` have overloads supporting `std::basic_string_view`. Technically, these overloads accept types which are convertible to matching `std::basic_string_view` but not convertible to matching `const_pointer`. Give `folly::basic_fbstring` similar overloads.

Reviewed By: thedavekwon

Differential Revision: D62008263

fbshipit-source-id: 84cc357ec2acbba1e30eac84b2fdc42a76bc0f07

folly/FBString.h

@@ -1014,6 +1014,15 @@ class basic_fbstring {
  private:
   using string_view_type = std::basic_string_view<value_type, traits_type>;
 
+  template <typename StringViewLike>
+  static inline constexpr bool is_string_view_like_v =
+      std::is_convertible_v<StringViewLike const&, string_view_type> &&
+      !std::is_convertible_v<StringViewLike const&, const_pointer>;
+
+  template <typename StringViewLike, typename Dummy>
+  using if_is_string_view_like_t =
+      std::enable_if_t<is_string_view_like_v<StringViewLike>, Dummy>;
+
   static void procrustes(size_type& n, size_type nmax) {
     if (n > nmax) {
       n = nmax;
@@ -1109,19 +1118,13 @@ class basic_fbstring {
 
   template <
       typename StringViewLike,
-      std::enable_if_t<
-          std::is_convertible_v<const StringViewLike&, string_view_type> &&
-              !std::is_convertible_v<const StringViewLike&, const value_type*>,
-          int> = 0>
+      if_is_string_view_like_t<StringViewLike, int> = 0>
   explicit basic_fbstring(const StringViewLike& view, const A& a = A())
       : basic_fbstring(string_view_type(view), a, string_view_ctor{}) {}
 
   template <
       typename StringViewLike,
-      std::enable_if_t<
-          std::is_convertible_v<const StringViewLike&, string_view_type> &&
-              !std::is_convertible_v<const StringViewLike&, const value_type*>,
-          int> = 0>
+      if_is_string_view_like_t<StringViewLike, int> = 0>
   basic_fbstring(
       const StringViewLike& view, size_type pos, size_type n, const A& a = A())
       : basic_fbstring(
@@ -1282,6 +1285,14 @@ class basic_fbstring {
     return *this;
   }
 
+  template <
+      typename StringViewLike,
+      if_is_string_view_like_t<StringViewLike, int> = 0>
+  basic_fbstring& operator+=(const StringViewLike& like) {
+    append(like);
+    return *this;
+  }
+
   basic_fbstring& append(const basic_fbstring& str);
 
   basic_fbstring& append(
@@ -1305,6 +1316,14 @@ class basic_fbstring {
     return append(il.begin(), il.end());
   }
 
+  template <
+      typename StringViewLike,
+      if_is_string_view_like_t<StringViewLike, int> = 0>
+  basic_fbstring& append(const StringViewLike& like) {
+    string_view_type view = like;
+    return append(view.begin(), view.end());
+  }
+
   void push_back(const value_type c) { // primitive
     store_.push_back(c);
   }

folly/test/FBStringTest.cpp

@@ -331,6 +331,11 @@ void clause11_21_4_5(String& test) {
 
 template <class String>
 void clause11_21_4_6_1(String& test) {
+  using string_view_type = std::basic_string_view<
+      typename String::value_type,
+      typename String::traits_type>;
+  auto orig = test;
+
   // 21.3.5 modifiers (+=)
   String test1;
   randomString(&test1);
@@ -384,10 +389,23 @@ void clause11_21_4_6_1(String& test) {
   // initializer_list
   initializer_list<typename String::value_type> il{'a', 'b', 'c'};
   test += il;
+  // string_view
+  auto testsv = orig;
+  testsv += string_view_type{orig};
+  EXPECT_EQ(orig + orig, testsv);
+  // like string_view
+  auto testlsv = orig;
+  testlsv += invocable_to([&] { return string_view_type{orig}; });
+  EXPECT_EQ(orig + orig, testlsv);
 }
 
 template <class String>
 void clause11_21_4_6_2(String& test) {
+  using string_view_type = std::basic_string_view<
+      typename String::value_type,
+      typename String::traits_type>;
+  auto orig = test;
+
   // 21.3.5 modifiers (append, push_back)
   String s;
 
@@ -415,6 +433,14 @@ void clause11_21_4_6_2(String& test) {
   // initializer_list
   initializer_list<typename String::value_type> il{'a', 'b', 'c'};
   test.append(il);
+  // string_view
+  auto testsv = orig;
+  testsv.append(string_view_type{orig});
+  EXPECT_EQ(orig + orig, testsv);
+  // like string_view
+  auto testlsv = orig;
+  testlsv.append(folly::invocable_to([&] { return string_view_type{orig}; }));
+  EXPECT_EQ(orig + orig, testlsv);
 }
 
 template <class String>