Proposed specification for gsl::dyn_array (#2261)

* final draft of proposal

* fix alloc formal

* resolve ambiguity and fix TODO

* some wording improvements

* apply some feedback after offline review w/ @GabrielDosReis

* Update specification after review from C++ Core Guidelines

- Make sure iterators and references cannot be invalidated by any
  operations (besides destruction).
- Update specification to annotate that some functions require certain
  language features (ranges and concepts).
- Remove dependence on "Container" named requirements because they
  require iterator-invalidating behavior.
- Explicitly add default and move constructors.
- Fix some typos

* Update dyn_array documentation reflecting editors' feedback

Clarified that `gsl::dyn_array` cannot be moved or copied. Updated the FAQ section to emphasize that it is a fixed-size array without iterator/pointer-invalidating operations.

* Update FAQ regarding gsl::dyn_array usage

Clarified the purpose of gsl::dyn_array in the FAQ section.

---------

Co-authored-by: Gabriel Dos Reis <GabrielDosReis@users.noreply.github.com>

docs/dyn_array.md

@@ -0,0 +1,112 @@
+<!-- $ pandoc -V geometry:margin=1in -V colorlinks=true -o dyn_array.pdf dyn_array.md --> 
+
+# `gsl::dyn_array<T, Allocator>`
+
+`gsl::dyn_array` is a dynamic array that is intended to be a replacement for slinging
+around raw pointers and sizes. Notably, it _owns_ all of its elements. It should replace
+the following idioms:
+
+* Replace `new T[n]` with `gsl::dyn_array<T>(n)`.
+* Replace both `foo(T*, size_t)` and `foo(unique_ptr<T[]>&, size_t)` with
+`foo(gsl::dyn_array<T>&)`.
+
+It can be thought of like a...
+
+* `std::array` except the size is specified at runtime.
+* `std::vector` except it can neither shrink nor grow.
+
+By design, `gsl::dyn_array` is not a `Container` as defined by the C++ Named
+Requirements because we want to avoid the invalidation of iterators or references to
+`gsl::dyn_array` objects.  Furthermore, by design, there is no support for operations
+(other than destruction) that can invalidate iterators or pointers into the sequence
+owned by a `gsl::dyna_array` object. An `gsl::dyn_array` object cannot be moved,
+nor can it be copied.
+
+### Construction
+`gsl::dyn_array`s can be constructed in the following ways:
+
+* Default construct a `dyn_array` with no elements:
+```c++
+constexpr dyn_array();
+```
+
+* Move construct a `dyn_array` from `other`: not possible
+```c++
+dyn_array(dyn_array&& other) = delete;
+```
+
+* Construct a `dyn_array` with `n` default constructed elements:
+```c++
+constexpr explicit dyn_array(size_t n, const Allocator & alloc = Allocator());
+```
+
+* Construct a `dyn_array` with `n` elements, each copy constructed from `arg`:
+```c++
+constexpr dyn_array(size_t n, const T& arg, const Allocator & alloc = Allocator());
+``` 
+
+* Construct a `dyn_array` with elements from the range `[first, last)`:
+```c++
+template <typename InputIt>
+#ifdef __cpp_lib_concepts
+    requires(std::input_iterator<InputIt>)
+#endif /* __cpp_lib_concepts */
+constexpr dyn_array(InputIt first, InputIt last, const Allocator & alloc = Allocator());
+```
+
+* Construct a `dyn_array` from a range:
+```c++
+#ifdef __cpp_lib_containers_range
+template <typename R>
+    requires(std::ranges::range<R>)
+constexpr dyn_array(std::from_range_t, R&& r, const Allocator & alloc = Allocator());
+#endif /* __cpp_lib_containers_range */
+```
+
+* Construct a `dyn_array` with elements from the initializer list:
+```c++
+constexpr dyn_array(std::initializer_list<T>, const Allocator & alloc = Allocator());
+```
+
+### Operations
+In addition to the operations required by the named requirements, `gsl::dyn_array` will
+support the following operations:
+
+* Access the specified element **_with bounds checking_**:
+```c++
+constexpr T& operator[](size_t);
+constexpr const T& operator[](size_t) const;
+```
+
+* Access the underlying array:
+```c++
+constexpr T* data() noexcept;
+constexpr const T* data() const noexcept;
+```
+ 
+* Return the number of elements in the `dyn_array`:
+```c++
+constexpr size_t size() const noexcept;
+```
+
+### FAQ
+
+#### Why no push_back (and friends)?
+`gsl::dyn_array` is intended to be a fixed-size array and all objects should be
+constructed at creation.  It supports no iterator/pointer-invalidating operation.
+
+#### Why does `gsl::dyn_array` not conform to the `Container` Named Requirements?
+`gsl::dyn_array` is intended to be a safer replacement for raw pointers and sizes. We
+don't want users to accidentally use it in a way that would be unsafe. For example,
+`gsl::dyn_array` does not have copy or move operations. This is because it
+would be possible to invalidate existing iterators and references.
+
+### Bounds Checking Semantics
+If an out-of-bounds access (read or write) is attempted, `gsl::dyn_array` should follow
+the contract violation strategy outlined in [GSL.assert: Assertions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#gslassert-assertions).
+
+### References
+* [C++ Named Requirements](https://en.cppreference.com/w/cpp/named_req)
+* [Microsoft/GSL #1169](https://github.com/microsoft/GSL/issues/1169)
+* [isocpp/CppCoreGuidelines #2244](https://github.com/isocpp/CppCoreGuidelines/issues/2244)
+* [n3662](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3662.html)