MSVC: Hide unpack_detect in class

Author: AntoinePrv

cpp/src/arrow/util/bit_stream_utils_internal.h

@@ -181,6 +181,20 @@ class BitReader {
   }
 
  private:
+  template <typename T>
+
+  struct unpack_detect {
+    using type = std::make_unsigned_t<T>;
+  };
+
+  template <>
+  struct unpack_detect<bool> {
+    using type = uint8_t;
+  };
+
+  template <typename T>
+  using unpack_t = typename unpack_detect<T>::type;
+
   const uint8_t* buffer_;
   int max_bytes_;
 
@@ -296,18 +310,6 @@ inline bool BitReader::GetValue(int num_bits, T* v) {
   return GetBatch(num_bits, v, 1) == 1;
 }
 
-namespace internal {
-template <typename T>
-struct unpack_detect {
-  using type = std::make_unsigned_t<T>;
-};
-
-template <>
-struct unpack_detect<bool> {
-  using type = uint8_t;
-};
-}  // namespace internal
-
 template <typename T>
 inline int BitReader::GetBatch(int num_bits, T* v, int batch_size) {
   ARROW_DCHECK(buffer_ != NULL);
@@ -335,10 +337,9 @@ inline int BitReader::GetBatch(int num_bits, T* v, int batch_size) {
     }
   }
 
-  using unpack_t = typename internal::unpack_detect<T>::type;
-
-  int num_unpacked = ::arrow::internal::unpack(
-      buffer + byte_offset, reinterpret_cast<unpack_t*>(v + i), batch_size - i, num_bits);
+  int num_unpacked = ::arrow::internal::unpack(buffer + byte_offset,
+                                               reinterpret_cast<unpack_t<T>*>(v + i),
+                                               batch_size - i, num_bits);
   i += num_unpacked;
   byte_offset += num_unpacked * num_bits / 8;