<algorithm>/<iterator>: contiguous iterators with non-ptrdiff_t difference types

[foonathan]

Describe the bug

We're having a contiguous iterator whose difference_type is int. STL algorithms then translate it to pointers and back, but doing so they e.g. call the iterator's operator- with a std::ptrdiff_t, not difference_type, like here:

STL/stl/inc/xutility

Line 4944 in 3c1f3d7

return _Dest - (_LastPtr - _FirstPtr);

This causes a truncation warning.

Command-line test case

https://godbolt.org/z/YbojbWPMd

Expected behavior

No warning should happen, the implementation should cast to the appropriate difference type before invoking the operator.

STL version

Microsoft Visual Studio Professional 2022
Version 17.1.0

[frederick-vs-ja]

We have a number of issues on difference types. I guess there're also remain issues in <vector> , <deque>, and <xstring>.
CC @hewillk.

Edit: deque is likely to be fixed in #4049.

[hewillk]

The issue occurs with _Copy_memmove too:

STL/stl/inc/xutility

Line 4636 in 3c1f3d7

return _Dest + (_LastPtr - _FirstPtr);

Although I think the contiguous_iterator's difference type should at least have a width greater than or equal to ptrdiff_t.
testcase: https://godbolt.org/z/Tqhbs7j9v

[foonathan]

We still run into this problem with std::copy_n: https://godbolt.org/z/Gvq7Pfr7E

[StephanTLavavej]

@foonathan - Please open a new bug; commenting on closed bugs is hard to keep track of.

[frederick-vs-ja]

We still run into this problem with std::copy_n: https://godbolt.org/z/Gvq7Pfr7E

It seems that the "latest" version used by Godbolt is currently 17.11.x, while the bugfix was merged for 17.12 Preview 3. So perhaps you need to update VS to get the bugfix at this moment.

[CaseyCarter]

We still run into this problem with std::copy_n: https://godbolt.org/z/Gvq7Pfr7E

Fixed by #5046.