Merge 2019-07 CWG Motion 20

P1771R1 [[nodiscard]] for constructors

Fixes #3000.

Author: zygoloid

source/declarations.tex

@@ -9004,12 +9004,26 @@
 or with different \grammarterm{attribute-argument-clause}s)
 are allowed.
 
+\pnum
+A \defnadj{nodiscard}{type} is
+a (possibly cv-qualified) class or enumeration type
+marked \tcode{nodiscard} in a reachable declaration.
+A \defnadj{nodiscard}{call} is either
+\begin{itemize}
+\item
+  a function call expression\iref{expr.call}
+  that calls a function declared \tcode{nodiscard} in a reachable declaration or
+  whose return type is a nodiscard type, or
+\item
+  an explicit type
+  conversion~(\ref{expr.static.cast}, \ref{expr.cast}, \ref{expr.type.conv})
+  that constructs an object through a constructor declared \tcode{nodiscard}, or
+  that initializes an object of a nodiscard type.
+\end{itemize}
+
 \pnum
 \begin{note}
-A nodiscard call is a function call expression that
-calls a function previously declared \tcode{nodiscard}, or
-whose return type is a possibly cv-qualified class or enumeration type
-marked \tcode{nodiscard}. Appearance of a nodiscard call as
+Appearance of a nodiscard call as
 a potentially-evaluated discarded-value expression\iref{expr.prop}
 is discouraged unless explicitly cast to \tcode{void}.
 Implementations should issue a warning in such cases.
@@ -9024,10 +9038,22 @@
 \pnum
 \begin{example}
 \begin{codeblock}
+struct [[nodiscard]] my_scopeguard { @\commentellip@ };
+struct my_unique {
+  my_unique() = default;                                // does not acquire resource
+  [[nodiscard]] my_unique(int fd) { @\commentellip@ }         // acquires resource
+  ~my_unique() noexcept { @\commentellip@ }                   // releases resource, if any
+  @\commentellip@
+};
 struct [[nodiscard]] error_info { @\commentellip@ };
 error_info enable_missile_safety_mode();
 void launch_missiles();
 void test_missiles() {
+  my_scopeguard();              // warning encouraged
+  (void)my_scopeguard(),        // warning not encouraged, cast to \tcode{void}
+    launch_missiles();          // comma operator, statement continues
+  my_unique(42);                // warning encouraged
+  my_unique();                  // warning not encouraged
   enable_missile_safety_mode(); // warning encouraged
   launch_missiles();
 }