https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120386
Bug ID: 120386
Summary: std::unique_copy uses the output type for comparisons
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
#include <algorithm>
struct X { X(int){} };
void uniq(const int* first, const int* last, X* out)
{
std::unique_copy(first, last, out);
}
/usr/include/c++/14/bits/predefined_ops.h:117:23: error: no match for
‘operator==’ (operand types are ‘X’ and ‘const int’)
117 | { return *__it1 == *__it2; }
| ~~~~~~~^~~~~~~~~
When the output iterator has category std::forward_iterator_tag we choose an
implementation that compares the output value with the input value, even though
they might be different types.
The standard is clear (since C++98) that the comparisons should be on two
values from the input range:
Copies only the first element from every consecutive group of equal elements
referred
to by the iterator i in the range [first, last) for which the following
corresponding
conditions hold: *i == *(i - 1) or pred(*i, *(i - 1)) != false
We seem to have done this wrong since the beginning of time (at least since
2001).
