https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92102
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2019-10-15
Ever confirmed|0 |1
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced:
template<typename>
concept nope = false;
template<typename>
concept sure_thing = true;
template<typename T>
struct category
{ };
template<typename T> requires (!nope<T>)
struct category<T>
{ };
template<typename T> requires (!nope<T>) && sure_thing<T>
struct category<T>
{ using type = T; };
category<int>::type t;
ambig.cc:19:14: error: ambiguous template instantiation for 'struct
category<int>'
19 | category<int>::type t;
| ^~
ambig.cc:12:10: note: candidates are: 'template<class T> requires !(nope<T>)
struct category<T> [with T = int]'
12 | struct category<T>
| ^~~~~~~~~~~
ambig.cc:16:10: note: 'template<class T> requires !(nope<T>)
&& (sure_thing<T>) struct category<T> [with T = int]'
16 | struct category<T>
| ^~~~~~~~~~~
ambig.cc:19:16: error: invalid use of incomplete type 'struct category<int>'
19 | category<int>::type t;
| ^~~~
ambig.cc:8:10: note: declaration of 'struct category<int>'
8 | struct category
| ^~~~~~~~
To make subsumption of (!E) work I need to add a concept for !E
template<typename T>
concept nope_nope = !nope<T>;
and then use that:
template<typename T> requires nope_nope<T>
struct category<T>
{ };
template<typename T> requires nope_nope<T> && sure_thing<T>
struct category<T>
{ using type = T; };
