https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111158
Bug ID: 111158
Summary: diagnostics, colors, and std::same_as
Product: gcc
Version: 13.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Consider this broken example:
#include <concepts>
template <typename T>
struct Optional { };
auto f() -> Optional<int&>;
auto g()
#ifdef CONCEPTS
-> std::same_as<Optional<int>> auto
#else
-> Optional<int>
#endif
{
return f();
}
The code is wrong, g() is specifying it returns Optional<int> but actually
returns Optional<int&> and there's no conversion.
Without concepts (just using a normal trailing return type), we get:
<source>: In function 'Optional<int> g()':
<source>:15:13: error: could not convert 'f()' from 'Optional<int&>' to
'Optional<int>'
15 | return f();
| ~^~
| |
| Optional<int&>
Compiler returned: 1
Pasting it here like this actually undersells how good the error it is, because
it's actually in color, and the "int&" and "int" parts are a different color
from the Optinoal<...> part, so it really visually stands out. Very nice, gcc
devs!
With concepts (the -> std::same_as<Optional<int>> auto approach), we get:
<source>: In function 'auto [requires std::same_as<<placeholder>, Optional<int>
>] g()':
<source>:15:13: error: deduced return type does not satisfy placeholder
constraints
15 | return f();
| ~^~
<source>:15:13: note: constraints not satisfied
In file included from <source>:1:
/opt/compiler-explorer/gcc-trunk-20230824/include/c++/14.0.0/concepts:57:15:
required for the satisfaction of '__same_as<_Tp, _Up>' [with _Tp =
Optional<int&>; _Up = Optional<int>]
/opt/compiler-explorer/gcc-trunk-20230824/include/c++/14.0.0/concepts:62:13:
required for the satisfaction of 'same_as<auto [requires
std::same_as<<placeholder>, Optional<int> >], Optional<int> >' [with auto
[requires std::same_as<<placeholder>, Optional<int> >] = Optional<int&>]
/opt/compiler-explorer/gcc-trunk-20230824/include/c++/14.0.0/concepts:57:32:
note: the expression 'is_same_v<_Tp, _Up> [with _Tp = Optional<int&>; _Up =
Optional<int>]' evaluated to 'false'
57 | concept __same_as = std::is_same_v<_Tp, _Up>;
| ~~~~~^~~~~~~~~~~~~~~~~~~
This isn't colored as nicely, and in general the diagnostic is I think quite a
bit worse - it contains a lot of information that simply isn't relevant to the
user. Like the fact that std::same_as is specified in this weird way in terms
of this other __same_as thing. That's an implementation detail that pretty much
never matters.
It'd be nice if gcc had dedicated diagnostics handling for std::same_as<T, U>
and just wrote that T and U are different types and applied the same kind of
color highlighting that it does for the non-concepts case - which helped me a
lot to visually call out that the distinction was int vs int&.
Link to compiler explorer, where you can see the colors with gcc 13.2:
https://godbolt.org/z/YaKjzvM98
