Using a partial-concept-id in a template-introduction

[CaseyCarter]

The grammar allows the use of a partial-concept-id in a template-introduction, e.g.,

template <class T, class U> concept bool Common() { return /* */ }

Common<int>{T}
auto f(T t) { return t != 0 ? t : 42; }

which I would expect to be equivalent to

template <class T>
  requires Common<T, int>()
auto f(T t) { return t != 0 ? t : 42; }

However, the rules for concept resolution in [temp.constr.resolve]/3 don't properly specify behavior for a concept name used in a partial-concept-id in a template-introduction. Bullet 3.2 is the only one concerned with template-introductions:

• If C is the concept-name in a template-introduction. the concept argument list is a sequence
  of wildcards of the same length as the introduction-list of the template-introduction.

to allow for use of a partial-concept-id in a template-introduction, I think this needs to be replaced with:

• If C is part of a template-introduction,
  • If the template-introduction consists solely of the concept-name C, the concept argument list is a sequence of wildcards of the same length as the introduction-list of the template-introduction.
  • If C is the concept-name of a partial-concept-id, the concept argument list is a sequence of wildcards of the same length as the introduction-list of the template-introduction followed by the arguments that appear in the template-argument-list of the partial-concept-id.

[CaseyCarter]

I suppose the obvious alternative solution is to change the grammar of template-introduction to forbid _partial-concept-id_s; something like

template-introduction: nested-name-specifier_opt concept-name { introduction-list }.

Apologies if this isn't the correct place to report issues with the TS, I haven't heard anything about the to-be-created issue repository. Let me know if it would be more appropriate to post this to c++std-core.

[asutton]

This is fine. The issues list will be a published document (I still need to
create it).

I don't think I'd ever anticipated a partial-concept-id in that position.
Disallowing it seems reasonable.

Andrew Sutton

On Tue, Sep 15, 2015 at 11:37 PM, Casey Carter [email protected]
wrote:

I suppose the obvious alternative solution is to change the grammar of
template-introduction to forbid _partial-concept-id_s; something like

template-introduction: nested-name-specifieropt concept-name {
introduction-list }.

Apologies if this isn't the correct place to report issues with the TS, I
haven't heard anything about the to-be-created issue repository. Let me
know if it would be more appropriate to post this to c++std-core.

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