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Section: 21.3.3 [meta.type.synop] Status: Resolved Submitter: Russia Opened: 2016-11-09 Last modified: 2018-11-12
Priority: 2
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Discussion:
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.
[2016-11-09, Jonathan provides wording]
[Issues Telecon 16-Dec-2016]
Priority 2
[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.
[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
[2017-06-15 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'
[2018-08 Batavia Monday issue discussion]
Issues 2797, 2939, 3022, and 3099 are all closely related. Walter to write a paper resolving them.
[2018-11-11 Resolved by P1285R0, adopted in San Diego.]
Proposed resolution:
This wording is relative to N4606.
Add a new paragraph after 21.3.5.4 [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 21.3.6 [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 21.3.7 [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 21.3.8 [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.