Precondition on is_convertible is too strong
Casey Carter

Created on 2018-12-09.00:00:00, last changed 2018-12-21.15:26:00.


Date: 2018-12-21.15:26:00

Proposed resolution:

This wording is relative to N4791.

  1. Modify Table 49 in [meta.rel] as follows:

    Table 49 — Type relationship predicates
    Template Condition Comments
    […] […] […]
    template<class From, class To>
    struct is_­convertible;
    see below From and To shall be a
    complete types, an
    arrays of unknown bound,
    or cv void types.
    template<class From, class To>
    struct is_nothrow_­convertible;
    is true and the
    conversion, as defined by
    is_­convertible, is known
    not to throw any
    exceptions ([expr.unary.noexcept])
    From and To shall be a
    complete types, an
    arrays of unknown bound,
    or cv void types.
    […] […] […]
Date: 2018-12-21.00:00:00

[ 2018-12-21 Reflector prioritization ]

Set Priority to 3

Date: 2018-12-09.00:00:00

Per Table 49 in [meta.rel], the preconditions for both is_convertible<From, To> and is_nothrow_convertible<From, To> are:

From and To shall be complete types, arrays of unknown bound, or cv void types.

Consequently, this program fragment:

struct S;
static_assert(is_convertible_v<S, const S&>);

has undefined behavior despite that the actual behavior of is_convertible specified in [meta.rel]/5:

-5- The predicate condition for a template specialization is_­convertible<From, To> shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:

To test() {
  return declval<From>();

[ Note: …]

is well-formed: declval<S>() is an xvalue of type S, which certainly does implicitly convert to const S&. We should relax the precondition to allow this perfectly valid case (and similar cases like is_convertible<S, S&&>), letting the cases that would in fact be invalid fall through to the blanket "incompletely-defined object type" wording in [meta.rqmts]/5.

Date User Action Args
2018-12-21 15:26:00adminsetmessages: + msg10264
2018-12-10 00:30:01adminsetmessages: + msg10254
2018-12-09 00:00:00admincreate