https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108645
Bug ID: 108645
Summary: Change in behavior, std::accumulate doesn't always
work as expected in C++20 builds
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: eteran at alum dot rit.edu
Target Milestone: ---
I encountered an interesting change in behavior today involving playing with
std::move_iterator types using the following code:
```
#include <vector>
#include <iostream>
#include <string>
#include <numeric>
#include <iterator>
void print_v(const char *rem, const std::vector<std::string> &v) {
std::cout << rem;
for (const std::string &s : v)
std::cout << '"' << s << '"' << ' ';
std::cout << '\n';
}
int main() {
std::vector<std::string> v = {"this", "_", "is", "_", "an", "_",
"example"};
print_v("Old contents of the vector: ", v);
std::string concat =
std::accumulate(std::make_move_iterator(v.begin()),
std::make_move_iterator(v.end()),
std::string());
print_v("New contents of the vector: ", v);
std::cout << "Concatenated as string: " << '"' << concat << '"' <<
'\n';
}
```
The expected output is:
```
Old contents of the vector: "this" "_" "is" "_" "an" "_" "example"
New contents of the vector: "" "" "" "" "" "" ""
Concatenated as string: "this_is_an_example"
```
And that is the output I get when compiling with `-std=c++17`. So the
expectation is that the moved from strings become empty.
However, when using C++20, where `std::accumulate` is now defined to use
`std::move`, the output is this instead:
```
Old contents of the vector: "this" "_" "is" "_" "an" "_" "example"
New contents of the vector: "this" "_" "is" "_" "an" "_" "example"
Concatenated as string: "this_is_an_example"
```
A few thoughts:
1. I'm not sure this is a "bug" since moved from objects are in an unspecified,
valid state. Being "unchanged" seems to fit that description just fine.
2. I'm guessing that this is a weird situation where the `operator+` overload
that has a `basic_string &&` and the lhs isn't stealing the guts as expected
for some reason.
Tested with both clang and gcc trunk, so it seems to be a library level issue.
C++20 Example: https://godbolt.org/z/7MMqWbxEr
C++17 Example: https://godbolt.org/z/6YnrrE5zf
