[dcl.type.auto.deduct] Clarify initializer for placeholder type deduction.

Author: jensmaurer

source/declarations.tex

@@ -1709,8 +1709,6 @@
 each of which shall
 be followed by a non-empty
 \grammarterm{initializer}.
-If the \grammarterm{initializer} is a parenthesized \grammarterm{expression-list},
-the \grammarterm{expression-list} shall be a single \grammarterm{assignment-expression}.
 \begin{example}
 \begin{codeblock}
 auto x = 5;                     // OK: \tcode{x} has type \tcode{int}
@@ -1896,45 +1894,64 @@
 
 \pnum
 A type \tcode{T} containing a placeholder type,
-and a corresponding initializer $E$,
+and a corresponding \grammarterm{initializer-clause} $E$,
 are determined as follows:
 \begin{itemize}
 \item
-for a non-discarded \tcode{return} statement that occurs
+For a non-discarded \tcode{return} statement that occurs
 in a function declared with a return type
 that contains a placeholder type,
-\tcode{T} is the declared return type
-and $E$ is the operand of the \tcode{return} statement.
-If the \tcode{return} statement
-has no operand,
-then $E$ is \tcode{void()};
+\tcode{T} is the declared return type.
+\begin{itemize}
+\item
+If the \tcode{return} statement has no operand,
+then $E$ is \tcode{void()}.
+\item
+If the operand is a \grammarterm{braced-init-list}\iref{dcl.init.list},
+the program is ill-formed.
+\item
+If the operand is an \grammarterm{expression} $X$
+that is not an \grammarterm{assignment-expression},
+$E$ is \tcode{($X$)}.
+\begin{note}
+A comma expression\iref{expr.comma} is not
+an \grammarterm{assignment-expression}.
+\end{note}
 \item
-for a variable declared with a type
+Otherwise, $E$ is the operand of the \tcode{return} statement.
+\end{itemize}
+If $E$ has type \keyword{void},
+\tcode{T} shall be either
+\opt{\grammarterm{type-constraint}} \tcode{decltype(auto)} or
+\cv{}~\opt{\grammarterm{type-constraint}} \keyword{auto}.
+\item
+For a variable declared with a type
 that contains a placeholder type,
-\tcode{T} is the declared type of the variable
-and $E$ is the initializer.
-If the initialization is direct-list-initialization,
-the initializer shall be a \grammarterm{braced-init-list}
-containing only a single \grammarterm{assignment-expression}
-and $E$ is the \grammarterm{assignment-expression};
+\tcode{T} is the declared type of the variable.
+\begin{itemize}
 \item
-for a non-type template parameter declared with a type
+If the initializer of the variable is a \grammarterm{brace-or-equal-initializer}
+of the form \tcode{= \grammarterm{initializer-clause}},
+$E$ is the \grammarterm{initializer-clause}.
+\item
+If the initializer is a \grammarterm{braced-init-list},
+it shall consist of a single brace-enclosed \grammarterm{initializer-clause}
+and the \grammarterm{initializer-clause} shall be
+an \grammarterm{assignment-expression},
+and $E$ is the \grammarterm{assignment-expression}.
+\item
+If the initializer is a parenthesized \grammarterm{expression-list},
+the \grammarterm{expression-list} shall be
+a single \grammarterm{assignment-expression}
+and $E$ is the \grammarterm{assignment-expression}.
+\end{itemize}
+\item
+For a non-type template parameter declared with a type
 that contains a placeholder type,
 \tcode{T} is the declared type of the non-type template parameter
 and $E$ is the corresponding template argument.
 \end{itemize}
 
-In the case of a \tcode{return} statement with no operand
-or with an operand of type \keyword{void},
-\tcode{T} shall be either
-\opt{\grammarterm{type-constraint}} \tcode{decltype(auto)} or
-\cv{}~\opt{\grammarterm{type-constraint}} \keyword{auto}.
-
-\pnum
-If the deduction is for a \tcode{return} statement
-and $E$ is a \grammarterm{braced-init-list}\iref{dcl.init.list},
-the program is ill-formed.
-
 \pnum
 If the \grammarterm{placeholder-type-specifier} is of the form
 \opt{\grammarterm{type-constraint}} \keyword{auto},