Merge 2018-06 LWG Motion 20

Fixes #2137

Author: zygoloid

source/utilities.tex

@@ -15334,6 +15334,7 @@
 \rSec2[meta.type.synop]{Header \tcode{<type_traits>} synopsis}
 
 \indexhdr{type_traits}%
+\indexlibrary{\idxcode{type_identity_t}}%
 \begin{codeblock}
 namespace std {
   // \ref{meta.help}, helper class
@@ -15503,6 +15504,7 @@
     using add_pointer_t    = typename add_pointer<T>::type;
 
   // \ref{meta.trans.other}, other transformations
+  template<class T> struct type_identity;
   template<size_t Len, size_t Align = @\textit{default-alignment}@> // see \ref{meta.trans.other}
     struct aligned_storage;
   template<size_t Len, class... Types> struct aligned_union;
@@ -15514,6 +15516,8 @@
   template<class T> struct underlying_type;
   template<class Fn, class... ArgTypes> struct invoke_result;
 
+  template<class T>
+    using type_identity_t   = typename type_identity<T>::type;
   template<size_t Len, size_t Align = @\textit{default-alignment}@> // see \ref{meta.trans.other}
     using aligned_storage_t = typename aligned_storage<Len, Align>::type;
   template<size_t Len, class... Types>
@@ -16847,6 +16851,12 @@
 \lhdr{Template}   &   \rhdr{Comments} \\ \capsep
 \endhead
 
+\indexlibrary{\idxcode{type_identity}}%
+\tcode{template<class T>\br
+ struct type_identity;}
+ &
+ The member typedef \tcode{type} names the type \tcode{T}. \\ \rowsep
+
 \indexlibrary{\idxcode{aligned_storage}}%
 \tcode{template<size_t Len,\br
  size_t Align\br