https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94673
Bug ID: 94673
Summary: [concepts] What is the return type of local parameters
of requires expressions?
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc-bugs at marehr dot dialup.fu-berlin.de
Target Milestone: ---
What should I expect when I write the following concept:
```c++
#include <utility>
#include <type_traits>
template <typename t, typename u>
concept same_as = std::is_same_v<t, u>;
template <typename t>
concept foo = requires(t v)
{
{v} -> same_as<t>;
};
static_assert(foo<int>); // gcc 9 / clang
static_assert(foo<int &>); // clang
static_assert(foo<int const &>); // clang
```
https://godbolt.org/z/rRkCzE
* clang says that the type of the expression `v` is `t`.
* gcc 9 partially agreed on this for `foo<int>`.
* gcc 10 rejects all of them
* msvc has the same opinion as gcc10, but also fails to provide an answer why.
gcc10 gives me an (improvable?) reason why the concept failed:
```
<source>:13:15: note: constraints not satisfied
<source>:8:9: required by the constraints of 'template<class t> concept foo'
<source>:8:15: in requirements with 'int v'
<source>:10:6: note: 'v' does not satisfy return-type-requirement
```
