Deduction guide for fbvector (#1090)

Summary:
- Add deduction guide for `fbvector` which is only available if the
  feature test macro for deduction guides is available or if we are on
  MSVC.
Pull Request resolved: https://github.com/facebook/folly/pull/1090

Reviewed By: nbronson

Differential Revision: D14663586

Pulled By: yfeldblum

fbshipit-source-id: b3db8fc6ee3b13b84bb6a4a49d2a1801dc80fd4a

folly/FBVector.h

@@ -1748,4 +1748,12 @@ void attach(fbvector<T, A>& v, T* data, size_t sz, size_t cap) {
   v.impl_.z_ = data + cap;
 }
 
+#if __cpp_deduction_guides >= 201703
+template <
+    class InputIt,
+    class Allocator =
+        std::allocator<typename std::iterator_traits<InputIt>::value_type>>
+fbvector(InputIt, InputIt, Allocator = Allocator())
+    ->fbvector<typename std::iterator_traits<InputIt>::value_type, Allocator>;
+#endif
 } // namespace folly

folly/test/FBVectorTest.cpp

@@ -256,3 +256,15 @@ TEST(FBVector, zero_len) {
   fb6 = il;
   fbvector<int> fb7(fb6.begin(), fb6.end());
 }
+
+#if __cpp_deduction_guides >= 201703
+TEST(FBVector, deduction_guides) {
+  fbvector<int> v(3);
+
+  fbvector x(v.begin(), v.end());
+  EXPECT_TRUE((std::is_same_v<fbvector<int>, decltype(x)>));
+
+  fbvector y{v.begin(), v.end()};
+  EXPECT_TRUE((std::is_same_v<fbvector<fbvector<int>::iterator>, decltype(y)>));
+}
+#endif