Proposed resolution:

This wording is relative to N4901.

[Drafting note: The proposed wording below contains also conditional changes, it is therefore depending upon a decision]

1. Modify [stacktrace.basic.overview] as indicated:

   -1- The class template basic_stacktrace satisfies the requirements of an allocator-aware container (Table 80 [tab:container.alloc.req]), a sequence container ([sequence.reqmts]), a contiguous container, and a reversible container ([container.requirements.general]) except that […]

2. Modify [stacktrace.basic.obs] as indicated:

   using const_iterator = implementation-defined;
   

   -1- The type models random_access_iteratorcontiguous_iterator ([iterator.concept.random.access][iterator.concept.contiguous]) and meets the Cpp17RandomAccessIterator requirements ([random.access.iterators]).

Optional additional changes (the following parts are proposed only if data() is wanted)

3. Modify [stacktrace.basic.overview], class template basic_stacktrace synopsis, as indicated:

   […]
   const_reference operator[](size_type) const;
   const_reference at(size_type) const;
   
   const stacktrace_entry* data() const noexcept;
   
   // [stacktrace.basic.cmp], comparisons
   […]
   

4. Modify [stacktrace.basic.obs] as indicated:

   const_reference at(size_type frame_no) const;
   

   -13- […]

   -14- […]

   const stacktrace_entry* data() const noexcept;
   

   -?- Returns: frames_.data().

[ 2022-01-29; Reflector poll ]

Set priority to 3 after reflector poll.
"The problem here is that a handful of member functions (operator[], at, perhaps begin) expose references to the vector elements directly, which can be read to require contiguity. We should rephrase the members at issue to not do that."

Currently std::basic_stacktrace has an exposition-only std::vector member for storing its elements. According to [objects.within.classes]/3, it seems that it is effectively required that elements of a std::basic_stacktrace are contiguously stored. However, the implication seems not used otherwhere. The iterator type of a std::basic_stacktrace is only required to be random access iterator.

IMO if it is required that std::basic_stacktrace uses contiguous storage, we should explicitly strengthen some requirements, perhaps a the member function data should be added.