- move add_const_from from traits.h to its own header (closes #27)

- move `add_reference_from` from `traits.h` to its own header (closes #28)
- move `constify` from `traits.h` to its own header (closes #29)
- move `constify_from` from `traits.h` to its own header (closes #30)
- move `data_member_getter` from `traits.h` to its own header (closes #31)
- move `remove_rvalue_reference` from `traits.h` to its own header (closes #32)
- move `same_reference_as` from `traits.h` to its own header (closes #33)

fatal/type/add_const_from.h

@@ -0,0 +1,47 @@
+/*
+ *  Copyright (c) 2016, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#ifndef FATAL_INCLUDE_fatal_type_add_const_from_h
+#define FATAL_INCLUDE_fatal_type_add_const_from_h
+
+#include <type_traits>
+
+namespace fatal {
+
+/**
+ * Applies `std::add_const` to a type iff some other type is const.
+ *
+ * Example:
+ *
+ *  struct foo {};
+ *
+ *  // yields `foo`
+ *  using result1 = add_const_from<foo, int>::type;
+ *
+ *  // yields `foo const`
+ *  using result2 = add_const_from<foo, int const>::type;
+ *
+ *  // yields `foo const`
+ *  using result3 = add_const_from<foo const, int const>::type;
+ *
+ * @author: Marcelo Juchem <marcelo@fb.com>
+ */
+template <typename T, typename>
+struct add_const_from {
+  using type = T;
+};
+
+template <typename T, typename TFrom>
+struct add_const_from<T, TFrom const> {
+  using type = typename std::add_const<T>::type;
+};
+
+} // namespace fatal
+
+#endif // FATAL_INCLUDE_fatal_type_add_const_from_h

fatal/type/add_reference_from.h

@@ -0,0 +1,64 @@
+/*
+ *  Copyright (c) 2016, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#ifndef FATAL_INCLUDE_fatal_type_add_reference_from_h
+#define FATAL_INCLUDE_fatal_type_add_reference_from_h
+
+#include <type_traits>
+
+namespace fatal {
+
+/**
+ * Given types `T` and `U`:
+ * - if `U` is not a reference, yield `T`
+ * - if `U` is an l-value reference, yield `std::add_lvalue_reference<T>::type`
+ * - if `U` is an r-value reference, yield `std::add_rvalue_reference<T>::type`
+ *
+ * Example:
+ *
+ *  struct foo {};
+ *
+ *  // yields `foo`
+ *  using result1 = add_reference_from<foo, int>::type;
+ *
+ *  // yields `foo &&`
+ *  using result2 = add_reference_from<foo &&, int>::type;
+ *
+ *  // yields `foo &`
+ *  using result3 = add_reference_from<foo, int &>::type;
+ *
+ *  // yields `foo &`
+ *  using result4 = add_reference_from<foo &&, int &>::type;
+ *
+ *  // yields `foo &&`
+ *  using result5 = add_reference_from<foo, int &&>::type;
+ *
+ *  // yields `foo &`
+ *  using result6 = add_reference_from<foo &, int &&>::type;
+ *
+ * @author: Marcelo Juchem <marcelo@fb.com>
+ */
+template <typename T, typename>
+struct add_reference_from {
+  using type = T;
+};
+
+template <typename T, typename TFrom>
+struct add_reference_from<T, TFrom &> {
+  using type = typename std::add_lvalue_reference<T>::type;
+};
+
+template <typename T, typename TFrom>
+struct add_reference_from<T, TFrom &&> {
+  using type = typename std::add_rvalue_reference<T>::type;
+};
+
+} // namespace fatal
+
+#endif // FATAL_INCLUDE_fatal_type_add_reference_from_h

fatal/type/constify.h

@@ -0,0 +1,54 @@
+/*
+ *  Copyright (c) 2016, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#ifndef FATAL_INCLUDE_fatal_type_constify_h
+#define FATAL_INCLUDE_fatal_type_constify_h
+
+#include <fatal/type/add_reference_from.h>
+
+#include <type_traits>
+
+namespace fatal {
+
+/**
+ * Adds the const qualifier to a type.
+ *
+ * If the type is a reference, to some other type `U`,
+ * make it the same kind of reference to `U const`.
+ *
+ * Example:
+ *
+ *  // yields `int const`
+ *  using result1 = constify<int>::type;
+ *
+ *  // yields `int const &`
+ *  using result2 = constify<int &>::type;
+ *
+ *  // yields `int const &&`
+ *  using result3 = constify<int &&>::type;
+ *
+ *  // yields `int const &`
+ *  using result4 = constify<int const &>::type;
+ *
+ *  // yields `int *const`
+ *  using result4 = constify<int *>::type;
+ *
+ * @author: Marcelo Juchem <marcelo@fb.com>
+ */
+template <typename T>
+struct constify {
+  using type = typename add_reference_from<
+    typename std::decay<T>::type const,
+    T
+  >::type;
+};
+
+} // namespace fatal
+
+#endif // FATAL_INCLUDE_fatal_type_constify_h

fatal/type/constify_from.h

@@ -0,0 +1,57 @@
+/*
+ *  Copyright (c) 2016, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#ifndef FATAL_INCLUDE_fatal_type_constify_from_h
+#define FATAL_INCLUDE_fatal_type_constify_from_h
+
+#include <fatal/type/constify.h>
+
+namespace fatal {
+
+///////////////////
+// constify_from //
+///////////////////
+
+/**
+ * Applies `constify` to a type iff some other type is const.
+ *
+ * Example:
+ *
+ *  struct foo {};
+ *
+ *  // yields `foo const`
+ *  using result1 = constify_from<foo, int const>::type;
+ *
+ *  // yields `foo const &`
+ *  using result2 = constify_from<foo &, int const>::type;
+ *
+ *  // yields `foo &&`
+ *  using result3 = constify_from<foo &&, int>::type;
+ *
+ *  // yields `foo const &`
+ *  using result4 = constify_from<foo const &, int const>::type;
+ *
+ *  // yields `foo const &`
+ *  using result5 = constify_from<foo const &, int>::type;
+ *
+ * @author: Marcelo Juchem <marcelo@fb.com>
+ */
+template <typename T, typename>
+struct constify_from {
+  using type = T;
+};
+
+template <typename T, typename TFrom>
+struct constify_from<T, TFrom const> {
+  using type = typename constify<T>::type;
+};
+
+} // namespace fatal
+
+#endif // FATAL_INCLUDE_fatal_type_constify_from_h