https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103239
Bug ID: 103239
Summary: confusing template argument deduction error message
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ich.freak at gmx dot net
Target Milestone: ---
When trying to resolve a function call, GCC iterates through the candidates,
checking if it can deduce/substitute template parameters. When failing to
deduce/substitute, it outputs nicely why it failed to do so. However, when 2
candidates fail for the same reason, it appears that this reason is only
printed for the first candidate, not for the second one, leaving the user with
a confusing message such as
[...]
note: template argument deduction/substitution failed:
<source>:8:5: note: candidate: [...]
that is, no reason is printed after the colon and, instead, the next candidate
is listed. This seems surprising and unintended. It would be nice to tell the
user why deduction/substitution failed for his/her favorite candidate.
I drafted a small example here: https://godbolt.org/z/PjeGa3YPe
The code is:
#include <iostream>
template<int> struct X {
using T = int;
template<class Y> using Z = Y::T;
X() = default;
template<class Y, class Q = Z<Y>> X(Y&& y, int, Q&& q = Q()) {}
template<class Y, class Q = Z<Y>>
X(Y&& y, int, Q&& q = Q(), int ignore = 10) {}
};
int main () {
const X<0> x = X<0>();
X<0> y(x, 0);
}
