Merge 2018-06 CWG Motion 3

Fixes #2115

Author: zygoloid

source/expressions.tex

@@ -1392,6 +1392,8 @@
 the entity is said to be \defnx{implicitly captured}{capture!implicit}
 by each intervening \grammarterm{lambda-expression} with an associated
 \grammarterm{capture-default} that does not explicitly capture it.
+The implicit capture of \tcode{*this} is deprecated when the
+\grammarterm{capture-default} is \tcode{=}; see \ref{depr.capture.this}.
 \begin{example}
 \begin{codeblock}
 void f(int, const int (&)[2] = {});         // \#1

source/future.tex

@@ -14,6 +14,25 @@
 An implementation may declare library names and entities described in this Clause with the
 \tcode{deprecated} attribute\iref{dcl.attr.deprecated}.
 
+\rSec1[depr.capture.this]{Implicit capture of \tcode{*this} by reference}
+
+\pnum
+For compatibility with prior \Cpp{} International Standards,
+a lambda expression with \grammarterm{capture-default}
+\tcode{=}\iref{expr.prim.lambda.capture} may implicitly capture
+\tcode{*this} by reference.
+\begin{example}
+\begin{codeblock}
+struct X {
+  int x;
+  void foo(int n) {
+    auto f = [=]() { x = n; };          // deprecated: \tcode{x} means \tcode{this->x}, not a copy thereof
+    auto g = [=, this]() { x = n; };    // recommended replacement
+  }
+};
+\end{codeblock}
+\end{example}
+
 \rSec1[depr.static_constexpr]{Redeclaration of \tcode{static constexpr} data members}
 
 \pnum