Merge 2018-06 CWG Motion 12

Fixes #2124

Author: zygoloid

source/compatibility.tex

@@ -1872,6 +1872,49 @@
 if those entities are only referenced in contexts
 that do not result in an odr-use.
 
+\rSec2[diff.cpp17.dcl.decl]{\ref{dcl.decl}: declarators}
+
+\diffref{dcl.init.aggr}
+\change A class that has user-declared constructors is never an aggregate.
+\rationale Remove potentially error-prone aggregate initialization
+which may apply notwithstanding the declared constructors of a class.
+\effect Valid \CppXVII{} code that aggregate-initializes
+a type with a user-declared constructor
+may be ill-formed or have different semantics
+in this International Standard.
+\begin{codeblock}
+struct A {              // not an aggregate; previously an aggregate
+  A() = delete;
+};
+
+struct B {              // not an aggregate; previously an aggregate
+  B() = default;
+  int i = 0;
+};
+
+struct C {              // not an aggregate; previously an aggregate
+  C(C&&) = default;
+  int a, b;
+};
+
+A a{};                  // ill-formed; previously well-formed
+B b = {1};              // ill-formed; previously well-formed
+auto* c = new C{2, 3};  // ill-formed; previously well-formed
+
+struct Y;
+
+struct X {
+  operator Y();
+};
+
+struct Y {              // not an aggregate; previously an aggregate
+  Y(const Y&) = default;
+  X x;
+};
+
+Y y{X{}};               // copy constructor call; previously aggregate-initialization
+\end{codeblock}
+
 \rSec2[diff.cpp17.special]{\ref{special}: special member functions}
 
 \diffrefs{class.ctor}{class.dtor}

source/declarators.tex

@@ -2953,7 +2953,7 @@
 An \defn{aggregate} is an array or a class\iref{class} with
 \begin{itemize}
 \item
-no user-provided, \tcode{explicit}, or inherited constructors\iref{class.ctor},
+no user-declared or inherited constructors\iref{class.ctor},
 \item
 no private or protected non-static data members\iref{class.access},
 \item
@@ -3456,7 +3456,7 @@
 \indextext{initialization!array of class objects}%
 \begin{note}
 An aggregate array or an aggregate class may contain elements of a
-class type with a user-provided constructor\iref{class.ctor}.
+class type with a user-declared constructor\iref{class.ctor}.
 Initialization of these aggregate objects is described in~\ref{class.expl.init}.
 \end{note}