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

[Louis Dionne via Phabricator via libcxx-commits]

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Those constructors are very easy to misuse -- one could easily think that
the size passed to the constructor is the size of the range to exhibit
from the subrange. Instead, it's a size hint and it's UB to get it wrong.
Hence, when it's cheap to compute the real size of the range, it's cheap
to make sure that the user didn't get it wrong.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D108827

Files:
  libcxx/include/__ranges/subrange.h


Index: libcxx/include/__ranges/subrange.h
===================================================================
--- libcxx/include/__ranges/subrange.h
+++ libcxx/include/__ranges/subrange.h
@@ -118,7 +118,11 @@
     constexpr subrange(__convertible_to_non_slicing<_Iter> auto __iter, _Sent __sent,
                        make_unsigned_t<iter_difference_t<_Iter>> __n)
       requires (_Kind == subrange_kind::sized)
-      : _Base(_VSTD::move(__iter), __sent, __n) { }
+      : _Base(_VSTD::move(__iter), __sent, __n)
+    {
+      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");
+    }
 
     template<__different_from<subrange> _Range>
       requires borrowed_range<_Range> &&


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