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Showing content from https://cplusplus.github.io/LWG/issue3174 below:

Precondition on is_convertible is too strong

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Section: 21.3.8 [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 21.3.8 [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 21.3.8 [meta.rel] as follows:

   Table 49 — Type relationship predicates Template Condition Comments […] […] […]

   template<class From, class To>
   struct is_convertible;
   

   see below From andTo shall be a
   complete type s, an
   array s 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 andTo shall be a
   complete type s, an
   array s of unknown bound,
   or cv void types. […] […] […]

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