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975. is_convertible cannot be instantiated for non-convertible types

Section: 21.3.8 [meta.rel] Status: C++11 Submitter: Daniel Krügler Opened: 2009-01-25 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 206

Related to 1114⁽ⁱ⁾.

The current specification of std::is_convertible (reference is draft N2798) is basically defined by 21.3.8 [meta.rel] p.4:

In order to instantiate the template is_convertible<From, To>, the following code shall be well formed:

template <class T>
  typename add_rvalue_reference<T>::type create();

To test() {
  return create<From>();
}

[Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]

The first sentence can be interpreted, that e.g. the expression

std::is_convertible<double, int*>::value

is ill-formed because std::is_convertible<double, int*> could not be instantiated, or in more general terms: The wording requires that std::is_convertible<X, Y> cannot be instantiated for otherwise valid argument types X and Y if X is not convertible to Y.

This semantic is both unpractical and in contradiction to what the last type traits paper N2255 proposed:

If the following test function is well formed code b is true, else it is false.

template <class T>
  typename add_rvalue_reference<T>::type create();

To test() {
  return create<From>();
}

[Note: This definition gives well defined results for reference types, void types, array types, and function types. — end note]

[ Post Summit: ]

Jens: Checking that code is well-formed and then returning true/false sounds like speculative compilation. John Spicer would really dislike this. Please find another wording suggesting speculative compilation.

Recommend Open.

[ Post Summit, Howard adds: ]

John finds the following wording clearer:

Template	Condition	Comments
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.

Given the following function prototype:

template <class T>
  typename add_rvalue_reference<T>::type create();

is_convertible<From, To>::value shall be true if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, else is_convertible<From, To>::value shall be false.

To test() {
  return create<From>();
}

Original proposed wording:

In 21.3.8 [meta.rel]/4 change:

In order to instantiate the template is_convertible<From, To>, the following code shall be well formed If the following code is well formed is_convertible<From, To>::value is true, otherwise false:[..]

Revision 2

In 21.3.8 [meta.rel] change:

Template	Condition	Comments
...	...	...
template <class From, class To> struct is_convertible;	The code set out below shall be well formed. see below	From and To shall be complete types, arrays of unknown bound, or (possibly cv-qualified) void types.

-4- In order to instantiate the template is_convertible<From, To>, the following code shall be well formed: Given the following function prototype:

template <class T> 
  typename add_rvalue_reference<T>::type create();

is_convertible<From, To>::value inherits either directly or indirectly from true_type if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, else is_convertible<From, To>::value inherits either directly or indirectly from false_type.

To test() { 
  return create<From>(); 
}

[Note: This requirement gives well defined results for reference types, void types, array types, and function types. -- end note]

[ Batavia (2009-05): ]

We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 21.3.8 [meta.rel] change:

Template	Condition	Comments
...	...	...
template <class From, class To> struct is_convertible;	The code set out below shall be well formed. see below	From and To shall be complete types, arrays of unknown bound, or (possibly cv-qualified) void types.

-4- In order to instantiate the template is_convertible<From, To>, the following code shall be well formed: Given the following function prototype:

template <class T> 
  typename add_rvalue_reference<T>::type create();

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 create<From>(); 
}

[Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]