Fix AlignedSysAllocator converting constructor

Summary:
`align()` was private, so the converting construct didn't have access
to `other.align()`.

Reviewed By: yfeldblum

Differential Revision: D9785137

fbshipit-source-id: e64a66eccd8a8b28db27873c0aaa7305b642c926

folly/Memory.h

@@ -409,6 +409,9 @@ class AlignedSysAllocator : private Align {
  private:
   using Self = AlignedSysAllocator<T, Align>;
 
+  template <typename, typename>
+  friend class AlignedSysAllocator;
+
   constexpr Align const& align() const {
     return *this;
   }

folly/test/MemoryTest.cpp

@@ -167,6 +167,13 @@ TEST(AlignedSysAllocator, bad_alloc_default) {
   }
 }
 
+TEST(AlignedSysAllocator, converting_constructor) {
+  using Alloc1 = AlignedSysAllocator<float>;
+  using Alloc2 = AlignedSysAllocator<double>;
+  Alloc1 const alloc1(1024);
+  Alloc2 const alloc2(alloc1);
+}
+
 TEST(allocate_sys_buffer, compiles) {
   auto buf = allocate_sys_buffer(256);
   //  Freed at the end of the scope.