https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92102
Bug ID: 92102
Summary: identical requires-expression not subsumed
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Blocks: 67491
Target Milestone: ---
This fails to compile with -std=gnu++2a:
template<typename T, typename U> constexpr bool same = false;
template<typename T> constexpr bool same<T, T> = true;
template<typename T, typename U>
concept same_same = same<T, U> && same<U, T>;
template<typename T>
struct traits { using type = T; };
struct tag { };
template<typename T>
struct category
{ };
template<typename T>
requires requires { typename traits<T>::type; }
struct category<T>
{ };
template<typename T>
requires requires { typename traits<T>::type; }
&& same_same<typename traits<T>::type, tag>
struct category<T>
{ using type = int; };
category<tag>::type t;
ambig.cc:27:14: error: ambiguous template instantiation for 'struct
category<tag>'
27 | category<tag>::type t;
| ^~
ambig.cc:18:10: note: candidates are: 'template<class T> requires
requires{typename traits<T>::type;} struct category<T> [with T = tag]'
18 | struct category<T>
| ^~~~~~~~~~~
ambig.cc:24:10: note: 'template<class T> requires
requires{typename traits<T>::type;} && (same_same<typename traits::type, tag>)
struct category<T> [with T = tag]'
24 | struct category<T>
| ^~~~~~~~~~~
ambig.cc:27:16: error: invalid use of incomplete type 'struct category<tag>'
27 | category<tag>::type t;
| ^~~~
ambig.cc:13:10: note: declaration of 'struct category<tag>'
13 | struct category
| ^~~~~~~~
It compiles OK if the subsumed constraint is a concept instead of a
requires-expression, even though that should be equivalent:
template<typename T, typename U> constexpr bool same = false;
template<typename T> constexpr bool same<T, T> = true;
template<typename T, typename U>
concept same_same = same<T, U> && same<U, T>;
template<typename T>
struct traits { using type = T; };
template<typename T>
concept traitsy = requires { typename traits<T>::type; };
struct tag { };
template<typename T>
struct category
{ };
template<typename T>
requires traitsy<T>
struct category<T>
{ };
template<typename T>
requires traitsy<T>
&& same_same<typename traits<T>::type, tag>
struct category<T>
{ using type = int; };
category<tag>::type t;
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67491
[Bug 67491] [meta-bug] concepts issues
