Merge 2019-07 CWG Motion 3

P1331R2 Permitting trivial default initialization in constexpr contexts

Fixes #2982.

Author: zygoloid

source/declarations.tex

@@ -828,8 +828,7 @@
 \item an identifier label\iref{stmt.label},
 \item a definition of a variable
 of non-literal type or
-of static or thread storage duration or
-for which no initialization is performed.
+of static or thread storage duration.
 \end{itemize}
 \begin{note}
 A \grammarterm{function-body} that is \tcode{= delete} or \tcode{= default}
@@ -853,8 +852,8 @@
   return value;
 }
 constexpr int uninit() {
-  int a;                        // error: variable is uninitialized
-  return a;
+  struct { int a; } s;
+  return s.a;                   // error: uninitialized read of \tcode{s.a}
 }
 constexpr int prev(int x)
   { return --x; }               // OK
@@ -886,10 +885,6 @@
 shall satisfy the requirements for a \grammarterm{function-body} of a
 constexpr function;
 
-\item
-every non-variant non-static data member and base class subobject
-shall be initialized\iref{class.base.init};
-
 \item
 if the class is a union having variant members\iref{class.union}, exactly one of them
 shall be initialized;

source/expressions.tex

@@ -6935,6 +6935,10 @@
 that is applied to a glvalue
 that refers to a non-active member of a union or a subobject thereof;
 
+\item
+an lvalue-to-rvalue conversion that is applied to
+an object with an indeterminate value\iref{basic.indet};
+
 \item
 an invocation of an implicitly-defined copy/move constructor or
 copy/move assignment operator

source/preprocessor.tex

@@ -1684,7 +1684,7 @@
 \defnxname{cpp_capture_star_this}                 & \tcode{201603L} \\ \rowsep
 \defnxname{cpp_char8_t}                           & \tcode{201811L} \\ \rowsep
 \defnxname{cpp_conditional_explicit}              & \tcode{201806L} \\ \rowsep
-\defnxname{cpp_constexpr}                         & \tcode{201603L} \\ \rowsep
+\defnxname{cpp_constexpr}                         & \tcode{201907L} \\ \rowsep
 \defnxname{cpp_coroutines}                        & \tcode{201902L} \\ \rowsep
 \defnxname{cpp_decltype}                          & \tcode{200707L} \\ \rowsep
 \defnxname{cpp_decltype_auto}                     & \tcode{201304L} \\ \rowsep