Merge 2022-02 CWG Motion 2

P2173R1 Attributes on lambda-expressions

Author: tkoeppe

source/declarations.tex

@@ -8600,12 +8600,11 @@
 dependency propagation into and out of functions.
 No
 \grammarterm{attribute-argument-clause} shall be present. The attribute may be
-applied to the \grammarterm{declarator-id} of a
-\grammarterm{parameter-declaration} in a function declaration or lambda, in
+applied to a parameter of a function or lambda, in
 which case it specifies that the initialization of the parameter carries a
 dependency to\iref{intro.multithread} each lvalue-to-rvalue
 conversion\iref{conv.lval} of that object. The attribute may also be applied
-to the \grammarterm{declarator-id} of a function declaration, in which case it
+to a function or a lambda call operator, in which case it
 specifies that the return value, if any, carries a dependency to the evaluation
 of the function call expression.
 
@@ -8894,8 +8893,8 @@
 
 \pnum
 The \grammarterm{attribute-token} \tcode{nodiscard}
-may be applied to the \grammarterm{declarator-id}
-in a function declaration or to the declaration of a class or enumeration.
+may be applied to a function or a lambda call operator or
+to the declaration of a class or enumeration.
 An \grammarterm{attribute-argument-clause} may be present
 and, if present, shall have the form:
 
@@ -8985,9 +8984,9 @@
 
 \pnum
 The \grammarterm{attribute-token} \tcode{noreturn} specifies that a function does not return.
-No
-\grammarterm{attribute-argument-clause} shall be present. The attribute may be applied to the
-\grammarterm{declarator-id} in a function declaration. The first declaration of a function shall
+No \grammarterm{attribute-argument-clause} shall be present.
+The attribute may be applied to a function or a lambda call operator.
+The first declaration of a function shall
 specify the \tcode{noreturn} attribute if any declaration of that function specifies the
 \tcode{noreturn} attribute. If a function is declared with the \tcode{noreturn} attribute in one
 translation unit and the same function is declared without the \tcode{noreturn} attribute in another

source/expressions.tex

@@ -1673,8 +1673,9 @@
 
 \begin{bnf}
 \nontermdef{lambda-expression}\br
-    lambda-introducer lambda-declarator compound-statement\br
-    lambda-introducer \terminal{<} template-parameter-list \terminal{>} \opt{requires-clause} lambda-declarator compound-statement
+    lambda-introducer \opt{attribute-specifier-seq} lambda-declarator compound-statement\br
+    lambda-introducer \terminal{<} template-parameter-list \terminal{>} \opt{requires-clause} \opt{attribute-specifier-seq}\br
+    \bnfindent lambda-declarator compound-statement
 \end{bnf}
 
 \begin{bnf}
@@ -1684,13 +1685,11 @@
 
 \begin{bnf}
 \nontermdef{lambda-declarator}\br
-    lambda-specifiers\br
-    \terminal{(} parameter-declaration-clause \terminal{)} lambda-specifiers \opt{requires-clause}
-\end{bnf}
-
-\begin{bnf}
-\nontermdef{lambda-specifiers}\br
-    \opt{lambda-specifier-seq} \opt{noexcept-specifier} \opt{attribute-specifier-seq} \opt{trailing-return-type}
+    lambda-specifier-seq \opt{noexcept-specifier} \opt{attribute-specifier-seq} \opt{trailing-return-type}\br
+    noexcept-specifier \opt{attribute-specifier-seq} \opt{trailing-return-type}\br
+    \opt{trailing-return-type}\br
+    \terminal{(} parameter-declaration-clause \terminal{)} \opt{lambda-specifier-seq} \opt{noexcept-specifier} \opt{attribute-specifier-seq}\br
+    \bnfindent \opt{trailing-return-type} \opt{requires-clause}
 \end{bnf}
 
 \begin{bnf}
@@ -1727,6 +1726,20 @@
 object\iref{function.objects}.
 \end{note}
 
+\pnum
+An ambiguity can arise
+because a \grammarterm{requires-clause} can end in
+an \grammarterm{attribute-specifier-seq},
+which collides with
+the \grammarterm{attribute-specifier-seq} in \grammarterm{lambda-expression}.
+In such cases,
+any attributes are treated as
+\grammarterm{attribute-specifier-seq} in \grammarterm{lambda-expression}.
+\begin{note}
+Such ambiguous cases cannot have valid semantics
+because the constraint expression would not have type \keyword{bool}.
+\end{note}
+
 \pnum
 A \grammarterm{lambda-specifier-seq}
 shall contain at most one of each \grammarterm{lambda-specifier} and
@@ -1740,13 +1753,15 @@
 \end{note}
 
 \pnum
-If a \grammarterm{lambda-expression} includes an empty
-\grammarterm{lambda-declarator}, it is as if the \grammarterm{lambda-declarator} were
-\tcode{()}.
-The lambda return type is \keyword{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}.
+If a \grammarterm{lambda-declarator} does not include
+a \grammarterm{parameter-declaration-clause},
+it is as if \tcode{()} were inserted
+at the start of the \grammarterm{lambda-declarator}.
+If the \grammarterm{lambda-declarator}
+does not include a \grammarterm{trailing-return-type},
+the lambda return type is \keyword{auto},
+which is deduced from \keyword{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}
@@ -1888,6 +1903,9 @@
 applies to the corresponding function call operator or operator template.
 An \grammarterm{attribute-specifier-seq} in a \grammarterm{lambda-declarator} appertains
 to the type of the corresponding function call operator or operator template.
+An \grammarterm{attribute-specifier-seq} in a \grammarterm{lambda-expression}
+preceding a \grammarterm{lambda-declarator}
+appertains to the corresponding function call operator or operator template.
 The function call operator or any given operator template specialization
 is a constexpr function if either
 the corresponding \grammarterm{lambda-expression}{'s}