Skip to content

[2026-03 CWG Motion 11] P4149R1 Define "immediate context" #8889

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
eisenwave wants to merge 1 commit into
base: main
Choose a base branch
from
+80 −64
Open

[2026-03 CWG Motion 11] P4149R1 Define "immediate context" #8889

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
9 changes: 4 additions & 5 deletions source/exceptions.tex
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -996,7 +996,9 @@
declaration (e.g., an explicit specialization or an overriding virtual
function);

\item the function is defined; or
\item the function is defined;

\item the function is represented by a reflection; or

\item the exception specification is needed for a defaulted
function that calls the function.
Expand All @@ -1009,10 +1011,7 @@
\end{note}
\end{itemize}
The exception specification of a defaulted
function is evaluated as described above only when needed; similarly, the
\grammarterm{noexcept-specifier} of a specialization
of a templated function
is instantiated only when needed.
function is evaluated as described above only when needed.
%
\indextext{exception specification|)}

Expand Down
135 changes: 76 additions & 59 deletions source/templates.tex
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -2677,31 +2677,30 @@
which shall be a class or variable template.

\pnum
A \defnadj{separately instantiated}{construct}
of a templated function \tcode{F} is a
\begin{itemize}
\item default argument,
\item \grammarterm{noexcept-specifier}, or
\item \grammarterm{function-contract-specifier}
\end{itemize}
of \tcode{F}.
For purposes of name lookup and instantiation,
default arguments,
\grammarterm{type-constraint}{s},
\grammarterm{requires-clause}{s}\iref{temp.pre},
and
\grammarterm{noexcept-specifier}{s}
of function templates
and
of member functions of class templates
separately instantiated constructs,
\grammarterm{type-constraint}{s}, and
\grammarterm{requires-clause}{s}\iref{temp.pre}
are considered definitions;
each
default argument,
\grammarterm{type-constraint},
\grammarterm{requires-clause},
or
\grammarterm{noexcept-specifier}
separately instantiated construct,
\grammarterm{type-constraint}, or
\grammarterm{requires-clause}
is a separate definition
which is unrelated
to the templated function definition or
to any other
default arguments,
\grammarterm{type-constraint}{s},
\grammarterm{requires-clause}{s},
or
\grammarterm{noexcept-specifier}{s}.
separately instantiated constructs,
\grammarterm{type-constraint}{s}, or
\grammarterm{requires-clause}{s}.
For the purpose of instantiation, the substatements of a constexpr if
statement\iref{stmt.if} are considered definitions.
For the purpose of name lookup and instantiation,
Expand Down Expand Up @@ -6643,19 +6642,6 @@
X<char> ch; // error: incomplete type \tcode{X<char>}
\end{codeblock}
\end{example}
\begin{note}
Within a template declaration,
a local class\iref{class.local} or enumeration and the members of
a local class are never considered to be entities that can be separately
instantiated (this includes their default arguments,
\grammarterm{noexcept-specifier}{s}, and non-static data member
initializers, if any,
but not their \grammarterm{type-constraint}{s} or \grammarterm{requires-clause}{s}).
As a result, the dependent names are looked up, the
semantic constraints are checked, and any templates used are instantiated as
part of the instantiation of the entity within which the local class or
enumeration is declared.
\end{note}

\pnum
The implicit instantiation of a class template specialization causes
Expand All @@ -6675,10 +6661,6 @@
unscoped member enumerations, and
member anonymous unions.
\end{itemize}
The implicit instantiation of a class template specialization
does not cause the implicit instantiation of
default arguments or \grammarterm{noexcept-specifier}{s}
of the class member functions.
\begin{example}
\begin{codeblock}
template<class T>
Expand Down Expand Up @@ -6742,11 +6724,6 @@
implicitly instantiated when it is referenced in a context that
requires a function definition to exist or
if the existence of the definition affects the semantics of the program.
Unless a call is to a function template explicit specialization or
to a member function of an explicitly specialized class template,
a default argument for a function template or a member function of a
class template is implicitly instantiated when the function is
called in a context that requires the value of the default argument.
\begin{note}
An inline function
that is the subject of an explicit instantiation declaration
Expand Down Expand Up @@ -6877,7 +6854,25 @@
causes specializations in the default member initializer to be instantiated.

\pnum
If a templated function
The separately instantiated constructs\iref{temp.decls.general}
of a templated function \tcode{F} that is either
\begin{itemize}
\item
a member of a local class or
\item
the function call operator of the closure type
of a non-generic lambda-expression
\end{itemize}
are instantiated when the declaration of \tcode{F} is instantiated.
\begin{note}
For the purposes of instantiation,
these constructs are still considered separately
from the function to which they belong.
\end{note}

\pnum
Other than as specified above,
if a templated function
\tcode{f}
is called in a way that requires a default argument to be used,
the dependent names are looked up, the semantics constraints are checked,
Expand All @@ -6896,7 +6891,9 @@
\tcode{f}.

\pnum
\begin{note}
Each default argument is instantiated independently.
\end{note}
\begin{example}
\begin{codeblock}
template<class T> void f(T x, T y = ydef(T()), T z = zdef(T()));
Expand All @@ -6914,16 +6911,16 @@
\end{example}

\pnum
The \grammarterm{noexcept-specifier} and \grammarterm{function-contract-specifier}s
of a function template specialization
are not instantiated along with the function declaration;
they are instantiated
when needed\iref{except.spec,dcl.contract.func}. If such a
specifier is needed but has not yet been
instantiated, the dependent names are looked up, the semantics constraints are
checked, and the instantiation of any template used in the
specifier is done as if it were being done as part
of instantiating the declaration of the specialization at that point.
Other than as specified above,
the separately instantiated constructs
of a specialization of a templated function
are instantiated only when needed\iref{except.spec,dcl.contract.func}.
When such a construct is needed but has not yet been instantiated,
the dependent names are looked up,
the semantics constraints are checked, and
the instantiation of any template used in the construct
is done as if it were being done as part of instantiating the declaration
of the specialization at that point.

\pnum
\begin{note}
Expand Down Expand Up @@ -8047,20 +8044,40 @@
\end{note}
Invalid types and expressions can result in a deduction failure
only in the immediate context of the deduction substitution loci.
The \defnadj{immediate}{context} of a deduction substitution locus
is that deduction substitution locus,
excluding bodies of \grammarterm{lambda-expression}s and separately instantiated constructs.
\begin{example}
\begin{codeblock}
template<typename T>
T* fun(T&& v); // \#1: the deduction substitution locus is the function type
// ``function of (rvalue reference to \tcode{T}) returning pointer to \tcode{T}''

void fun(...); // \#2

void test()
{
int i;
fun(i); // selects \#2 (forming the type ``pointer to reference to \tcode{int}'' fails in \#1)
}
\end{codeblock}
\end{example}
\begin{note}
Separately instantiated constructs are excluded from the immediate contexts
even if they are instantiated at the same time.
\end{note}
\begin{note}
The substitution into types and expressions can result
in effects such as the instantiation of class template specializations and/or
function template specializations, the generation of implicitly-defined functions,
etc. Such effects are not in the ``immediate context'' and can result in the
program being ill-formed.
in effects such as the instantiation of template specializations,
the generation of implicitly-defined functions, etc.
Such effects are not in the immediate context
and can result in the program being ill-formed.
\end{note}

\pnum
When substituting into a \grammarterm{lambda-expression},
substitution into its body is not in the immediate context.
\begin{note}
The intent is to avoid requiring implementations to deal with
substitution failure involving arbitrary statements.
No \grammarterm{statement} is ever in the immediate context
of a deduction substitution locus.
\begin{example}
\begin{codeblock}
template <class T>
Expand Down
Loading