3174. Precondition on is_convertible is too strong

Section: 21.3.7 [meta.rel] Status: New Submitter: Casey Carter Opened: 2018-12-09 Last modified: 2019-03-16

Priority: 3

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Discussion:

Per Table 49 in 21.3.7 [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.

[2018-12-21 Reflector prioritization]

Set Priority to 3

Proposed resolution:

This wording is relative to N4791.

  1. Modify Table 49 in 21.3.7 [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_convertible_v<From,
    To>     is true and the
    conversion, as defined by
    is_convertible, is known
    not to throw any
    exceptions (7.6.2.7 [expr.unary.noexcept])     		From and To shall be a
    complete types, an
    arrays of unknown bound,
    or cv void types.
    […] […] […]