https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92806
Bug ID: 92806
Summary: Suboptimal diagnostic for concept that depends on
non-bool value
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: diagnostic
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Blocks: 67491
Target Milestone: ---
template<typename T>
struct trait
{
enum { value = 1 };
};
template<typename T>
concept foo = trait<T>::value;
static_assert(foo<int>);
Compiled with -std=gnu++2a:
nonbool.cc:8:9: required by the constraints of 'template<class T> concept
foo'
nonbool.cc:10:24: error: constraint does not have type 'bool'
10 | static_assert(foo<int>);
| ^
nonbool.cc:10:15: error: non-constant condition for static assertion
10 | static_assert(foo<int>);
| ^~~~~~~~
nonbool.cc:8:9: required by the constraints of 'template<class T> concept
foo'
nonbool.cc:10:15: error: constraint does not have type 'bool'
It's not clear from the diagnostic that the problem is trait<T>::value on line
8, so the user is left thinking "how can a concept not have type 'bool'?!"
The fix is something like:
template<typename T>
concept foo = (bool)trait<T>::value;
^~~~~~
so it would be helpful to diagnose the non-bool type at the location of the
concept definition, not its use.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67491
[Bug 67491] [meta-bug] concepts issues
