https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72682
Bug ID: 72682
Summary: Redundant concept diagnostics when the same concept is
checked multiple times
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: adrian.wielgosik at gmail dot com
Target Milestone: ---
A simple code example:
template<typename T> using id=T;
template<typename T>
concept bool A = requires(T a) { a==a; };
template<typename T, typename U>
concept bool B =
A<T> && // (1) basic usage
A<T> && // (2) duplicate concept check
A<U> && // (3) different concept check that happens to check the same type
A<id<T>>; // (4) same as above
template<typename T> requires B<T, T> void f(T){}
struct S{} s;
int main(){ f(s); }
Produces the following diagnostic (first couple of lines that were not relevant
were omitted):
main.cpp:4:14: note: within ‘template<class T> concept const bool A<T> [with T
= S]’
concept bool A = requires(T a) { a==a; };
^
main.cpp:4:14: note: with ‘S a’
main.cpp:4:14: note: the required expression ‘(a == a)’ would be ill-formed
main.cpp:4:14: note: within ‘template<class T> concept const bool A<T> [with T
= S]’
main.cpp:4:14: note: with ‘S a’
main.cpp:4:14: note: the required expression ‘(a == a)’ would be ill-formed
main.cpp:4:14: note: within ‘template<class T> concept const bool A<T> [with T
= S]’
main.cpp:4:14: note: with ‘S a’
main.cpp:4:14: note: the required expression ‘(a == a)’ would be ill-formed
main.cpp:4:14: note: within ‘template<class T> concept const bool A<T> [with T
= S]’
main.cpp:4:14: note: with ‘S a’
main.cpp:4:14: note: the required expression ‘(a == a)’ would be ill-formed
Notice that all four checks in the end checked the concept A on the type S and
produced the exact same diagnostic.
For a real world example: if you use Ranges TS StrictTotallyOrdered<T, T> on
unorderable type, it's going to check StrictTotallyOrdered<T> three times on
the same type (and then also check all operators like "{t < u}->Boolean" extra
two times, and then a duplicate check of EqualityComparable), which can easily
produce over a dozen redundant messages, while ideally the whole diagnostic
should contain only six messages, one for each comparison operator.
