[libcxx-commits] [PATCH] D108827: [libc++] Add an assertion in the subrange constructors with a size hint

[Arthur O'Dwyer via Phabricator via libcxx-commits]

Quuxplusone requested changes to this revision.
Quuxplusone added inline comments.
This revision now requires changes to proceed.


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Comment at: libcxx/include/__ranges/subrange.h:123-125
+      if constexpr (sized_sentinel_for<_Sent, decltype(__iter)>)
+        _LIBCPP_ASSERT(ranges::distance(this->__begin_, this->__end_) == __n, "std::ranges::subrange was passed an invalid size hint");
+    }
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It is mildly sketchy that you're testing `sized_sentinel_for` on `decltype(__iter)` (which is not `_Iter`) but then invoking `ranges::distance` on `_Iter`. What if `!sized_sentinel_for<_Sent, _Iter>`?

Also, what if `!forward_iterator<_Iter>`? Then it is invalid to mess with `this->__begin_` except at the user's command.

Also, what if `!copyable<_Iter>`? I don't think that's possible today, but I think there are active proposals for move-only iterators. (But even with those proposals, I bet `forward_iterator<_Iter>` implies `copyable<_Iter>`. So that's probably fine.)

I suspect that this is a UB-if-violated "Precondition", not a "Mandates", on purpose, and we should just leave it alone.


Repository:
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  https://reviews.llvm.org/D108827/new/

https://reviews.llvm.org/D108827