Created on 2016-11-09.00:00:00 last changed 71 months ago
Proposed resolution:
This wording is relative to N4606.
Add a new paragraph after [meta.unary.prop] paragraph 3:
-?- If an instantiation of any template declared in this subclause fails to meet the preconditions, the program is ill-formed.
Change the specification for alignment_of in Table 39 in [meta.unary.prop.query]:
Table 39 — Type property queries template <class T> struct alignment_of; alignof(T).
Requires: alignof(T) shall be a valid expression (5.3.6), otherwise the program is ill-formed
Change the specification for is_base_of, is_convertible, is_callable, and is_nothrow_callable in Table 40 in [meta.rel]:
Table 40 — Type relationship predicates […] template <class Base, class
Derived>
struct is_base_of;[…] If Base and Derived are
non-union class types and are
different types (ignoring possible
cv-qualifiers) then Derived shall
be a complete type, otherwise the program is ill-formed.
[Note: Base classes that
are private, protected, or ambiguous are,
nonetheless, base classes. — end note]template <class From, class To>
struct is_convertible;see below From and To shall be complete
types, arrays of unknown bound,
or (possibly cv-qualified) void
types, otherwise the program is
ill-formed.template <class Fn, class...
ArgTypes, class R>
struct is_callable<
Fn(ArgTypes...), R>;[…] Fn, R, and all types in the
parameter pack ArgTypes shall
be complete types, (possibly
cv-qualified) void, or arrays of
unknown bound,
otherwise the program is ill-formed.template <class Fn, class...
ArgTypes, class R>
struct is_nothrow_callable<
Fn(ArgTypes...), R>;[…] Fn, R, and all types in the
parameter pack ArgTypes shall
be complete types, (possibly
cv-qualified) void, or arrays of
unknown bound,
otherwise the program is ill-formed.
Add a new paragraph after [meta.trans] paragraph 2:
-2- Each of the templates in this subclause shall be a TransformationTrait (20.15.1).
-?- If an instantiation of any template declared in this subclause fails to meet the Requires: preconditions, the program is ill-formed.
[ 2018-11-11 Resolved by P1285R0, adopted in San Diego. ]
[ 2017-06-15 request from Daniel ]
I don't believe that this should be "Ready"; I added the extra note to LWG 2797 *and* added the new issue 2939 exactly to *prevent* 2797 being accepted for C++17
Setting status back to 'Open'
[ 2017-03-03, Kona Friday morning ]
Unanimous consent to adopt this for C++17, but due to a misunderstanding, it wasn't on the ballot
Setting status to 'Ready' so we'll get it in immediately post-C++17
[ 2017-03-02, Daniel comments ]
LWG 2939 has been created to signal that some of our current type trait constraints are not quite correct and I recommend not to enforce the required diagnostics for traits that are sensitive to mismatches of the current approximate rules.
[ Issues Telecon 16-Dec-2016 ]
Priority 2
[ 2016-11-09, Jonathan provides wording ]
Addresses RU 2
Failed prerequirement for the type trait must result in ill-formed program. Otherwise hard detectable errors will happen:#include <type_traits> struct foo; void damage_type_trait() { // must be ill-formed std::is_constructible<foo, foo>::value; } struct foo{}; int main() { static_assert( // produces invalid result std::is_constructible<foo, foo>::value, "foo must be constructible from foo" ); }
Suggested resolution:
Add to the end of the [meta.type.synop] section:Program is ill-formed if precondition for the type trait is violated.
History | |||
---|---|---|---|
Date | User | Action | Args |
2018-11-12 04:30:58 | admin | set | messages: + msg10173 |
2018-11-12 04:30:58 | admin | set | status: open -> resolved |
2018-08-22 12:55:05 | admin | set | messages: + msg10092 |
2017-06-16 04:45:38 | admin | set | messages: + msg9269 |
2017-06-16 04:45:38 | admin | set | status: ready -> open |
2017-06-02 05:40:10 | admin | set | messages: + msg9192 |
2017-06-02 05:40:10 | admin | set | status: new -> ready |
2017-03-02 19:30:15 | admin | set | messages: + msg9043 |
2016-12-16 20:56:38 | admin | set | messages: + msg8727 |
2016-11-09 22:52:41 | admin | set | messages: + msg8604 |
2016-11-09 22:52:41 | admin | set | messages: + msg8603 |
2016-11-09 00:00:00 | admin | create |