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[expr.prim.lambda] Move operator() return type description #4069

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[expr.prim.lambda] Move operator() return type description #4069

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59 changes: 36 additions & 23 deletions source/expressions.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -1574,18 +1574,6 @@
If a \grammarterm{lambda-expression} does not include a
\grammarterm{lambda-declarator}, it is as if the \grammarterm{lambda-declarator} were
\tcode{()}.
The lambda return type is \tcode{auto}, which is replaced by the
type specified by the
\grammarterm{trailing-return-type} if provided and/or deduced from
\tcode{return} statements as described in~\ref{dcl.spec.auto}.
\begin{example}
\begin{codeblock}
auto x1 = [](int i){ return i; }; // OK: return type is \tcode{int}
auto x2 = []{ return { 1, 2 }; }; // error: deducing return type from \grammarterm{braced-init-list}
int j;
auto x3 = []()->auto&& { return j; }; // OK: return type is \tcode{int\&}
\end{codeblock}
\end{example}

\pnum
A lambda is a \defn{generic lambda}
Expand Down Expand Up @@ -1636,18 +1624,43 @@
The closure type for a \grammarterm{lambda-expression} has a public
inline function call operator (for a non-generic lambda) or
function call operator template (for a generic lambda)\iref{over.call}
whose parameters and return type
are described by the \grammarterm{lambda-expression}'s
\grammarterm{parameter-declaration-clause} and \grammarterm{trailing-return-type}
respectively, and whose
\grammarterm{template-parameter-list} consists of
the specified \grammarterm{template-parameter-list}, if any.
The \grammarterm{requires-clause} of the function call operator template
whose:
\begin{itemize}
\item \grammarterm{parameter-declaration-clause} is the \grammarterm{lambda-declarator}'s
\grammarterm{parameter-declaration-clause},

\item return type is \tcode{auto},
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I think that this isn't the return type, so much as it is the type in the decl-specifier-seq. (The declared return type is determined by the trailing-return-type if any, and the actual return type is what we deduce. Per [dcl.spec.auto.general]p3, the auto is just something that can appear in the decl-specifier-seq; it's not really a return type at all.) So:

Suggested change
\item return type is \tcode{auto},
\item \grammarterm{decl-specifier-seq} is the \grammarterm{lambda-declarator}'s \grammarterm{decl-specifier-seq} followed by \keyword{auto}, except that \keyword{constexpr} is also included if [...]

Also... should we say something about the attribute-specifier-seq?


\item \grammarterm{trailing-return-type} is the \grammarterm{lambda-declarator}'s
\grammarterm{trailing-return-type}, if any,

\item trailing \grammarterm{requires-clause} is the \grammarterm{lambda-declarator}'s
\grammarterm{requires-clause}, if any,

\item \grammarterm{template-parameter-list} is the \grammarterm{lambda-expression}'s
\grammarterm{template-parameter-list}, if any, and

\item \grammarterm{requires-clause} in the \grammarterm{template-head}
is the \grammarterm{requires-clause} immediately following
\tcode{<}~\grammarterm{template-parameter-list}{}~\tcode{>}, if any.
The trailing \grammarterm{requires-clause} of the function call operator
or operator template is the \grammarterm{requires-clause}
of the \grammarterm{lambda-declarator}, if any.
\tcode{<}~\grammarterm{template-parameter-list}{}~\tcode{>}
in the \grammarterm{lambda-expression}, if any.

\end{itemize}

\begin{note}
The actual return type will be the type specified by the
\grammarterm{trailing-return-type} if provided and/or deduced from
\tcode{return} statements as described in~\ref{dcl.spec.auto}.
\begin{example}
\begin{codeblock}
auto x1 = [](int i){ return i; }; // OK: return type is \tcode{int}
auto x2 = []{ return { 1, 2 }; }; // error: deducing return type from \grammarterm{braced-init-list}
int j;
auto x3 = []()->auto&& { return j; }; // OK: return type is \tcode{int\&}
\end{codeblock}
\end{example}
\end{note}

\begin{note}
The function call operator template for a generic lambda might be
an abbreviated function template\iref{dcl.fct}.
Expand Down