cmstd: Modernize CMake system headers

Provide a standardized way to handle the C++ "standard" headers
customized to be used with current CMake C++ standard constraints.
Offer under directory `cm` headers which can be used as direct
replacements of the standard ones.  For example:

    #include <cm/string_view>

can be used safely for CMake development in place of the `<string_view>`
standard header.

Fixes: #19491

9 files changed, 1094 insertions, 0 deletions

diff --git a/Utilities/std/.gitattributes b/Utilities/std/.gitattributes
new file mode 100644
index 0000000000..cd205494f3
--- /dev/null
+++ b/Utilities/std/.gitattributes
@@ -0,0 +1 @@
+cm/* our-c-style
diff --git a/Utilities/std/CMakeLists.txt b/Utilities/std/CMakeLists.txt
new file mode 100644
index 0000000000..63c0a60eae
--- /dev/null
+++ b/Utilities/std/CMakeLists.txt
@@ -0,0 +1,10 @@
+
+# source files for CMake std library
+set(SRCS cm/bits/string_view.cxx
+         cm/memory
+         cm/optional
+         cm/shared_mutex
+         cm/string_view
+         cm/utility)
+
+add_library(cmstd STATIC ${SRCS})
diff --git a/Utilities/std/cm/bits/string_view.cxx b/Utilities/std/cm/bits/string_view.cxx
new file mode 100644
index 0000000000..3b283da4aa
--- /dev/null
+++ b/Utilities/std/cm/bits/string_view.cxx
@@ -0,0 +1,301 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+
+#include <cm/string_view> // IWYU pragma: associated
+
+#ifndef CMake_HAVE_CXX_STRING_VIEW
+
+#  include "cm_kwiml.h"
+
+#  include <algorithm>
+#  include <ostream>
+#  include <stdexcept>
+
+namespace cm {
+
+string_view::const_reference string_view::at(size_type pos) const
+{
+  if (pos >= size_) {
+    throw std::out_of_range("Index out of range in string_view::at");
+  }
+  return data_[pos];
+}
+
+string_view::size_type string_view::copy(char* dest, size_type count,
+                                         size_type pos) const
+{
+  if (pos > size_) {
+    throw std::out_of_range("Index out of range in string_view::copy");
+  }
+  size_type const rcount = std::min(count, size_ - pos);
+  traits_type::copy(dest, data_ + pos, rcount);
+  return rcount;
+}
+
+string_view string_view::substr(size_type pos, size_type count) const
+{
+  if (pos > size_) {
+    throw std::out_of_range("Index out of range in string_view::substr");
+  }
+  size_type const rcount = std::min(count, size_ - pos);
+  return string_view(data_ + pos, rcount);
+}
+
+int string_view::compare(string_view v) const noexcept
+{
+  size_type const rlen = std::min(size_, v.size_);
+  int c = traits_type::compare(data_, v.data_, rlen);
+  if (c == 0) {
+    if (size_ < v.size_) {
+      c = -1;
+    } else if (size_ > v.size_) {
+      c = 1;
+    }
+  }
+  return c;
+}
+
+int string_view::compare(size_type pos1, size_type count1, string_view v) const
+{
+  return substr(pos1, count1).compare(v);
+}
+
+int string_view::compare(size_type pos1, size_type count1, string_view v,
+                         size_type pos2, size_type count2) const
+{
+  return substr(pos1, count1).compare(v.substr(pos2, count2));
+}
+
+int string_view::compare(const char* s) const
+{
+  return compare(string_view(s));
+}
+
+int string_view::compare(size_type pos1, size_type count1, const char* s) const
+{
+  return substr(pos1, count1).compare(string_view(s));
+}
+
+int string_view::compare(size_type pos1, size_type count1, const char* s,
+                         size_type count2) const
+{
+  return substr(pos1, count1).compare(string_view(s, count2));
+}
+
+string_view::size_type string_view::find(string_view v, size_type pos) const
+  noexcept
+{
+  for (; pos + v.size_ <= size_; ++pos) {
+    if (std::char_traits<char>::compare(data_ + pos, v.data_, v.size_) == 0) {
+      return pos;
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::find(char c, size_type pos) const noexcept
+{
+  return find(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::find(const char* s, size_type pos,
+                                         size_type count) const
+{
+  return find(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::find(const char* s, size_type pos) const
+{
+  return find(string_view(s), pos);
+}
+
+string_view::size_type string_view::rfind(string_view v, size_type pos) const
+  noexcept
+{
+  if (size_ >= v.size_) {
+    for (pos = std::min(pos, size_ - v.size_) + 1; pos > 0;) {
+      --pos;
+      if (std::char_traits<char>::compare(data_ + pos, v.data_, v.size_) ==
+          0) {
+        return pos;
+      }
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::rfind(char c, size_type pos) const noexcept
+{
+  return rfind(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::rfind(const char* s, size_type pos,
+                                          size_type count) const
+{
+  return rfind(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::rfind(const char* s, size_type pos) const
+{
+  return rfind(string_view(s), pos);
+}
+
+string_view::size_type string_view::find_first_of(string_view v,
+                                                  size_type pos) const noexcept
+{
+  for (; pos < size_; ++pos) {
+    if (traits_type::find(v.data_, v.size_, data_[pos])) {
+      return pos;
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::find_first_of(char c, size_type pos) const
+  noexcept
+{
+  return find_first_of(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::find_first_of(const char* s, size_type pos,
+                                                  size_type count) const
+{
+  return find_first_of(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::find_first_of(const char* s,
+                                                  size_type pos) const
+{
+  return find_first_of(string_view(s), pos);
+}
+
+string_view::size_type string_view::find_last_of(string_view v,
+                                                 size_type pos) const noexcept
+{
+  if (size_ > 0) {
+    for (pos = std::min(pos, size_ - 1) + 1; pos > 0;) {
+      --pos;
+      if (traits_type::find(v.data_, v.size_, data_[pos])) {
+        return pos;
+      }
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::find_last_of(char c, size_type pos) const
+  noexcept
+{
+  return find_last_of(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::find_last_of(const char* s, size_type pos,
+                                                 size_type count) const
+{
+  return find_last_of(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::find_last_of(const char* s,
+                                                 size_type pos) const
+{
+  return find_last_of(string_view(s), pos);
+}
+
+string_view::size_type string_view::find_first_not_of(string_view v,
+                                                      size_type pos) const
+  noexcept
+{
+  for (; pos < size_; ++pos) {
+    if (!traits_type::find(v.data_, v.size_, data_[pos])) {
+      return pos;
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::find_first_not_of(char c,
+                                                      size_type pos) const
+  noexcept
+{
+  return find_first_not_of(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::find_first_not_of(const char* s,
+                                                      size_type pos,
+                                                      size_type count) const
+{
+  return find_first_not_of(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::find_first_not_of(const char* s,
+                                                      size_type pos) const
+{
+  return find_first_not_of(string_view(s), pos);
+}
+
+string_view::size_type string_view::find_last_not_of(string_view v,
+                                                     size_type pos) const
+  noexcept
+{
+  if (size_ > 0) {
+    for (pos = std::min(pos, size_ - 1) + 1; pos > 0;) {
+      --pos;
+      if (!traits_type::find(v.data_, v.size_, data_[pos])) {
+        return pos;
+      }
+    }
+  }
+  return npos;
+}
+
+string_view::size_type string_view::find_last_not_of(char c,
+                                                     size_type pos) const
+  noexcept
+{
+  return find_last_not_of(string_view(&c, 1), pos);
+}
+
+string_view::size_type string_view::find_last_not_of(const char* s,
+                                                     size_type pos,
+                                                     size_type count) const
+{
+  return find_last_not_of(string_view(s, count), pos);
+}
+
+string_view::size_type string_view::find_last_not_of(const char* s,
+                                                     size_type pos) const
+{
+  return find_last_not_of(string_view(s), pos);
+}
+
+std::ostream& operator<<(std::ostream& o, string_view v)
+{
+  return o.write(v.data(), v.size());
+}
+
+std::string& operator+=(std::string& s, string_view v)
+{
+  s.append(v.data(), v.size());
+  return s;
+}
+}
+
+std::hash<cm::string_view>::result_type std::hash<cm::string_view>::operator()(
+  argument_type const& s) const noexcept
+{
+  // FNV-1a hash.
+  static KWIML_INT_uint64_t const fnv_offset_basis = 0xcbf29ce484222325;
+  static KWIML_INT_uint64_t const fnv_prime = 0x100000001b3;
+  KWIML_INT_uint64_t h = fnv_offset_basis;
+  for (char const& c : s) {
+    h = h ^ KWIML_INT_uint64_t(KWIML_INT_uint8_t(c));
+    h = h * fnv_prime;
+  }
+  return result_type(h);
+}
+#else
+// Avoid empty translation unit.
+void cm_string_view_cxx()
+{
+}
+#endif
diff --git a/Utilities/std/cm/iterator b/Utilities/std/cm/iterator
new file mode 100644
index 0000000000..083bce33d0
--- /dev/null
+++ b/Utilities/std/cm/iterator
@@ -0,0 +1,78 @@
+// -*-c++-*-
+// vim: set ft=cpp:
+
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cm_iterator
+#define cm_iterator
+
+#include <iterator> // IWYU pragma: export
+
+namespace cm {
+
+#if __cplusplus >= 201402L || defined(_MSVC_LANG) && _MSVC_LANG >= 201402L
+using std::make_reverse_iterator;
+
+using std::cbegin;
+using std::cend;
+#else
+template <class Iter>
+std::reverse_iterator<Iter> make_reverse_iterator(Iter it)
+{
+  return std::reverse_iterator<Iter>(it);
+}
+
+// std::c{begin,end} backport from C++14
+template <class C>
+#  if defined(_MSC_VER) && _MSC_VER < 1900
+auto cbegin(C const& c)
+#  else
+constexpr auto cbegin(C const& c) noexcept(noexcept(std::begin(c)))
+#  endif
+  -> decltype(std::begin(c))
+{
+  return std::begin(c);
+}
+
+template <class C>
+#  if defined(_MSC_VER) && _MSC_VER < 1900
+auto cend(C const& c)
+#  else
+constexpr auto cend(C const& c) noexcept(noexcept(std::end(c)))
+#  endif
+  -> decltype(std::end(c))
+{
+  return std::end(c);
+}
+#endif
+
+#if __cplusplus >= 201703L || defined(_MSVC_LANG) && _MSVC_LANG >= 201703L
+using std::size;
+#else
+
+// std::size backport from C++17.
+template <class C>
+#  if !defined(_MSC_VER) || _MSC_VER >= 1900
+constexpr
+#  endif
+  auto
+  size(C const& c) -> decltype(c.size())
+{
+  return c.size();
+}
+
+template <typename T, size_t N>
+#  if !defined(_MSC_VER) || _MSC_VER >= 1900
+constexpr
+#  endif
+  std::size_t
+  size(const T (&)[N]) throw()
+{
+  return N;
+}
+
+#endif
+
+} // namespace cm
+
+#endif
diff --git a/Utilities/std/cm/memory b/Utilities/std/cm/memory
new file mode 100644
index 0000000000..8ebded2779
--- /dev/null
+++ b/Utilities/std/cm/memory
@@ -0,0 +1,32 @@
+// -*-c++-*-
+// vim: set ft=cpp:
+
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cm_memory
+#define cm_memory
+
+#include <memory> // IWYU pragma: export
+#if !defined(CMake_HAVE_CXX_MAKE_UNIQUE)
+#  include <utility>
+#endif
+
+namespace cm {
+
+#if defined(CMake_HAVE_CXX_MAKE_UNIQUE)
+
+using std::make_unique;
+
+#else
+
+template <typename T, typename... Args>
+std::unique_ptr<T> make_unique(Args&&... args)
+{
+  return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
+}
+
+#endif
+
+} // namespace cm
+
+#endif
diff --git a/Utilities/std/cm/optional b/Utilities/std/cm/optional
new file mode 100644
index 0000000000..80b0951604
--- /dev/null
+++ b/Utilities/std/cm/optional
@@ -0,0 +1,344 @@
+// -*-c++-*-
+// vim: set ft=cpp:
+
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cm_optional
+#define cm_optional
+
+#if __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
+#  define CMake_HAVE_CXX_OPTIONAL
+#endif
+
+#if defined(CMake_HAVE_CXX_OPTIONAL)
+#  include <optional> // IWYU pragma: export
+#else
+#  include <memory>
+
+#  include <cm/utility>
+#endif
+
+namespace cm {
+
+#if defined(CMake_HAVE_CXX_OPTIONAL)
+
+using std::nullopt_t;
+using std::nullopt;
+using std::optional;
+using std::bad_optional_access;
+using std::make_optional;
+
+#else
+
+class bad_optional_access : public std::exception
+{
+  using std::exception::exception;
+};
+
+struct nullopt_t
+{
+  explicit constexpr nullopt_t(int) {}
+};
+
+constexpr nullopt_t nullopt{ 0 };
+
+template <typename T>
+class optional
+{
+public:
+  using value_type = T;
+
+  optional() noexcept = default;
+  optional(nullopt_t) noexcept;
+  optional(const optional& other);
+  optional(optional&& other) noexcept;
+
+  template <typename... Args>
+  explicit optional(cm::in_place_t, Args&&... args);
+
+  template <
+    typename U = T,
+    typename = typename std::enable_if<
+      std::is_constructible<T, U&&>::value &&
+      !std::is_same<typename std::decay<U>::type, cm::in_place_t>::value &&
+      !std::is_same<typename std::decay<U>::type,
+                    cm::optional<T>>::value>::type>
+  optional(U&& v);
+
+  ~optional();
+
+  optional& operator=(nullopt_t) noexcept;
+  optional& operator=(const optional& other);
+  optional& operator=(optional&& other) noexcept;
+
+  template <
+    typename U = T,
+    typename = typename std::enable_if<
+      !std::is_same<typename std::decay<U>::type, cm::optional<T>>::value &&
+      std::is_constructible<T, U>::value && std::is_assignable<T&, U>::value &&
+      (!std::is_scalar<T>::value ||
+       !std::is_same<typename std::decay<U>::type, T>::value)>::type>
+  optional& operator=(U&& v);
+
+  const T* operator->() const;
+  T* operator->();
+  const T& operator*() const&;
+  T& operator*() &;
+  const T&& operator*() const&&;
+  T&& operator*() &&;
+
+  explicit operator bool() const noexcept;
+  bool has_value() const noexcept;
+
+  T& value() &;
+  const T& value() const&;
+
+  T&& value() &&;
+  const T&& value() const&&;
+
+  template <typename U>
+  T value_or(U&& default_value) const&;
+
+  template <typename U>
+  T value_or(U&& default_value) &&;
+
+  void swap(optional& other) noexcept;
+  void reset() noexcept;
+
+  template <typename... Args>
+  T& emplace(Args&&... args);
+
+private:
+  bool _has_value = false;
+  std::allocator<T> _allocator;
+  union _mem_union
+  {
+    T value;
+
+    // Explicit constructor and destructor is required to make this work
+    _mem_union() noexcept {}
+    ~_mem_union() noexcept {}
+  } _mem;
+};
+
+template <typename T>
+optional<typename std::decay<T>::type> make_optional(T&& value)
+{
+  return optional<typename std::decay<T>::type>(std::forward<T>(value));
+}
+
+template <typename T, class... Args>
+optional<T> make_optional(Args&&... args)
+{
+  return optional<T>(in_place, std::forward<Args>(args)...);
+}
+
+template <typename T>
+optional<T>::optional(nullopt_t) noexcept
+{
+}
+
+template <typename T>
+optional<T>::optional(const optional& other)
+{
+  *this = other;
+}
+
+template <typename T>
+optional<T>::optional(optional&& other) noexcept
+{
+  *this = std::move(other);
+}
+
+template <typename T>
+template <typename... Args>
+optional<T>::optional(cm::in_place_t, Args&&... args)
+{
+  this->emplace(std::forward<Args>(args)...);
+}
+
+template <typename T>
+template <typename U, typename>
+optional<T>::optional(U&& v)
+{
+  this->emplace(std::forward<U>(v));
+}
+
+template <typename T>
+optional<T>::~optional()
+{
+  this->reset();
+}
+
+template <typename T>
+optional<T>& optional<T>::operator=(nullopt_t) noexcept
+{
+  this->reset();
+  return *this;
+}
+
+template <typename T>
+optional<T>& optional<T>::operator=(const optional& other)
+{
+  if (other.has_value()) {
+    if (this->has_value()) {
+      this->value() = *other;
+    } else {
+      this->emplace(*other);
+    }
+  } else {
+    this->reset();
+  }
+  return *this;
+}
+
+template <typename T>
+optional<T>& optional<T>::operator=(optional&& other) noexcept
+{
+  if (other.has_value()) {
+    if (this->has_value()) {
+      this->value() = std::move(*other);
+    } else {
+      this->emplace(std::move(*other));
+    }
+  } else {
+    this->reset();
+  }
+  return *this;
+}
+
+template <typename T>
+template <typename U, typename>
+optional<T>& optional<T>::operator=(U&& v)
+{
+  if (this->has_value()) {
+    this->value() = v;
+  } else {
+    this->emplace(std::forward<U>(v));
+  }
+  return *this;
+}
+
+template <typename T>
+const T* optional<T>::operator->() const
+{
+  return &**this;
+}
+
+template <typename T>
+T* optional<T>::operator->()
+{
+  return &**this;
+}
+
+template <typename T>
+const T& optional<T>::operator*() const&
+{
+  return this->_mem.value;
+}
+
+template <typename T>
+T& optional<T>::operator*() &
+{
+  return this->_mem.value;
+}
+
+template <typename T>
+const T&& optional<T>::operator*() const&&
+{
+  return std::move(**this);
+}
+
+template <typename T>
+T&& optional<T>::operator*() &&
+{
+  return std::move(**this);
+}
+
+template <typename T>
+bool optional<T>::has_value() const noexcept
+{
+  return this->_has_value;
+}
+
+template <typename T>
+optional<T>::operator bool() const noexcept
+{
+  return this->has_value();
+}
+
+template <typename T>
+T& optional<T>::value() &
+{
+  if (!this->has_value()) {
+    throw cm::bad_optional_access{};
+  }
+  return **this;
+}
+
+template <typename T>
+const T& optional<T>::value() const&
+{
+  if (!this->has_value()) {
+    throw cm::bad_optional_access{};
+  }
+  return **this;
+}
+
+template <typename T>
+template <typename U>
+T optional<T>::value_or(U&& default_value) const&
+{
+  return bool(*this) ? **this : static_cast<T>(std::forward<U>(default_value));
+}
+
+template <typename T>
+template <typename U>
+T optional<T>::value_or(U&& default_value) &&
+{
+  return bool(*this) ? std::move(**this)
+                     : static_cast<T>(std::forward<U>(default_value));
+}
+
+template <typename T>
+void optional<T>::swap(optional& other) noexcept
+{
+  if (this->has_value()) {
+    if (other.has_value()) {
+      using std::swap;
+      swap(**this, *other);
+    } else {
+      other.emplace(std::move(**this));
+      this->reset();
+    }
+  } else if (other.has_value()) {
+    this->emplace(std::move(*other));
+    other.reset();
+  }
+}
+
+template <typename T>
+void optional<T>::reset() noexcept
+{
+  if (this->has_value()) {
+    this->_has_value = false;
+    std::allocator_traits<std::allocator<T>>::destroy(this->_allocator,
+                                                      &**this);
+  }
+}
+
+template <typename T>
+template <typename... Args>
+T& optional<T>::emplace(Args&&... args)
+{
+  this->reset();
+  std::allocator_traits<std::allocator<T>>::construct(
+    this->_allocator, &**this, std::forward<Args>(args)...);
+  this->_has_value = true;
+  return this->value();
+}
+
+#endif
+}
+
+#endif
diff --git a/Utilities/std/cm/shared_mutex b/Utilities/std/cm/shared_mutex
new file mode 100644
index 0000000000..2ac9447dc7
--- /dev/null
+++ b/Utilities/std/cm/shared_mutex
@@ -0,0 +1,76 @@
+// -*-c++-*-
+// vim: set ft=cpp:
+
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cm_shared_mutex
+#define cm_shared_mutex
+
+#if __cplusplus >= 201402L || defined(_MSVC_LANG) && _MSVC_LANG >= 201402L
+#  define CMake_HAVE_CXX_SHARED_LOCK
+#endif
+#if __cplusplus >= 201703L || defined(_MSVC_LANG) && _MSVC_LANG >= 201703L
+#  define CMake_HAVE_CXX_SHARED_MUTEX
+#endif
+
+#if defined(CMake_HAVE_CXX_SHARED_LOCK)
+#  include <shared_mutex> // IWYU pragma: export
+#endif
+#if !defined(CMake_HAVE_CXX_SHARED_MUTEX)
+#  include "cm_uv.h"
+#endif
+
+namespace cm {
+#if defined(CMake_HAVE_CXX_SHARED_MUTEX)
+using std::shared_mutex;
+#else
+class shared_mutex
+{
+  uv_rwlock_t _M_;
+
+public:
+  using native_handle_type = uv_rwlock_t*;
+
+  shared_mutex() { uv_rwlock_init(&_M_); }
+  ~shared_mutex() { uv_rwlock_destroy(&_M_); }
+
+  shared_mutex(shared_mutex const&) = delete;
+  shared_mutex& operator=(shared_mutex const&) = delete;
+
+  void lock() { uv_rwlock_wrlock(&_M_); }
+  bool try_lock() { return uv_rwlock_trywrlock(&_M_) == 0; }
+  void unlock() { uv_rwlock_wrunlock(&_M_); }
+
+  void lock_shared() { uv_rwlock_rdlock(&_M_); }
+  void unlock_shared() { uv_rwlock_rdunlock(&_M_); }
+
+  native_handle_type native_handle() { return &_M_; }
+};
+#endif
+
+#if defined(CMake_HAVE_CXX_SHARED_LOCK)
+using std::shared_lock;
+#else
+template <typename T>
+class shared_lock
+{
+  T& _mutex;
+
+public:
+  using mutex_type = T;
+
+  shared_lock(T& m)
+    : _mutex(m)
+  {
+    _mutex.lock_shared();
+  }
+
+  ~shared_lock() { _mutex.unlock_shared(); }
+
+  shared_lock(shared_lock const&) = delete;
+  shared_lock& operator=(shared_lock const&) = delete;
+};
+#endif
+}
+
+#endif
diff --git a/Utilities/std/cm/string_view b/Utilities/std/cm/string_view
new file mode 100644
index 0000000000..4d359cb5c3
--- /dev/null
+++ b/Utilities/std/cm/string_view
@@ -0,0 +1,218 @@
+// -*-c++-*-
+// vim: set ft=cpp:
+
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cm_string_view
+#define cm_string_view
+
+#if __cplusplus >= 201703L || defined(_MSVC_LANG) && _MSVC_LANG >= 201703L
+#  define CMake_HAVE_CXX_STRING_VIEW
+#endif
+
+#ifdef CMake_HAVE_CXX_STRING_VIEW
+#  include <string_view> // IWYU pragma: export
+namespace cm {
+using std::string_view;
+}
+#else
+#  include <cstddef>
+#  include <functional>
+#  include <iosfwd>
+#  include <iterator>
+#  include <string>
+
+namespace cm {
+
+class string_view
+{
+public:
+  using traits_type = std::string::traits_type;
+  using value_type = char;
+  using pointer = char*;
+  using const_pointer = const char*;
+  using reference = char&;
+  using const_reference = char const&;
+  using const_iterator = const char*;
+  using iterator = const_iterator;
+  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+  using reverse_iterator = const_reverse_iterator;
+  using size_type = std::string::size_type;
+  using difference_type = std::string::difference_type;
+
+  static size_type const npos = static_cast<size_type>(-1);
+
+  string_view() noexcept = default;
+  string_view(string_view const&) noexcept = default;
+
+  string_view(const char* s, size_t count) noexcept
+    : data_(s)
+    , size_(count)
+  {
+  }
+
+  string_view(const char* s) noexcept
+    : data_(s)
+    , size_(traits_type::length(s))
+  {
+  }
+
+  // C++17 does not define this constructor.  Instead it defines
+  // a conversion operator on std::string to create a string_view.
+  // Since this implementation is used in C++11, std::string does
+  // not have that conversion.
+  string_view(std::string const& s) noexcept
+    : data_(s.data())
+    , size_(s.size())
+  {
+  }
+
+  // C++17 does not define this conversion.  Instead it defines
+  // a constructor on std::string that can take a string_view.
+  // Since this implementation is used in C++11, std::string does
+  // not have that constructor.
+  explicit operator std::string() const { return std::string(data_, size_); }
+
+  string_view& operator=(string_view const&) = default;
+
+  const_iterator begin() const noexcept { return data_; }
+  const_iterator end() const noexcept { return data_ + size_; }
+  const_iterator cbegin() const noexcept { return begin(); }
+  const_iterator cend() const noexcept { return end(); }
+
+  const_reverse_iterator rbegin() const noexcept
+  {
+    return const_reverse_iterator(end());
+  }
+  const_reverse_iterator rend() const noexcept
+  {
+    return const_reverse_iterator(begin());
+  }
+  const_reverse_iterator crbegin() const noexcept { return rbegin(); }
+  const_reverse_iterator crend() const noexcept { return rend(); }
+
+  const_reference operator[](size_type pos) const noexcept
+  {
+    return data_[pos];
+  }
+  const_reference at(size_type pos) const;
+  const_reference front() const noexcept { return data_[0]; }
+  const_reference back() const noexcept { return data_[size_ - 1]; }
+  const_pointer data() const noexcept { return data_; }
+
+  size_type size() const noexcept { return size_; }
+  size_type length() const noexcept { return size_; }
+  size_type max_size() const noexcept { return npos - 1; }
+  bool empty() const noexcept { return size_ == 0; }
+
+  void remove_prefix(size_type n) noexcept
+  {
+    data_ += n;
+    size_ -= n;
+  }
+  void remove_suffix(size_type n) noexcept { size_ -= n; }
+  void swap(string_view& v) noexcept
+  {
+    string_view tmp = v;
+    v = *this;
+    *this = tmp;
+  }
+
+  size_type copy(char* dest, size_type count, size_type pos = 0) const;
+  string_view substr(size_type pos = 0, size_type count = npos) const;
+
+  int compare(string_view v) const noexcept;
+  int compare(size_type pos1, size_type count1, string_view v) const;
+  int compare(size_type pos1, size_type count1, string_view v, size_type pos2,
+              size_type count2) const;
+  int compare(const char* s) const;
+  int compare(size_type pos1, size_type count1, const char* s) const;
+  int compare(size_type pos1, size_type count1, const char* s,
+              size_type count2) const;
+
+  size_type find(string_view v, size_type pos = 0) const noexcept;
+  size_type find(char c, size_type pos = 0) const noexcept;
+  size_type find(const char* s, size_type pos, size_type count) const;
+  size_type find(const char* s, size_type pos = 0) const;
+
+  size_type rfind(string_view v, size_type pos = npos) const noexcept;
+  size_type rfind(char c, size_type pos = npos) const noexcept;
+  size_type rfind(const char* s, size_type pos, size_type count) const;
+  size_type rfind(const char* s, size_type pos = npos) const;
+
+  size_type find_first_of(string_view v, size_type pos = 0) const noexcept;
+  size_type find_first_of(char c, size_type pos = 0) const noexcept;
+  size_type find_first_of(const char* s, size_type pos, size_type count) const;
+  size_type find_first_of(const char* s, size_type pos = 0) const;
+
+  size_type find_last_of(string_view v, size_type pos = npos) const noexcept;
+  size_type find_last_of(char c, size_type pos = npos) const noexcept;
+  size_type find_last_of(const char* s, size_type pos, size_type count) const;
+  size_type find_last_of(const char* s, size_type pos = npos) const;
+
+  size_type find_first_not_of(string_view v, size_type pos = 0) const noexcept;
+  size_type find_first_not_of(char c, size_type pos = 0) const noexcept;
+  size_type find_first_not_of(const char* s, size_type pos,
+                              size_type count) const;
+  size_type find_first_not_of(const char* s, size_type pos = 0) const;
+
+  size_type find_last_not_of(string_view v, size_type pos = npos) const
+    noexcept;
+  size_type find_last_not_of(char c, size_type pos = npos) const noexcept;
+  size_type find_last_not_of(const char* s, size_type pos,
+                             size_type count) const;
+  size_type find_last_not_of(const char* s, size_type pos = npos) const;
+
+private:
+  const char* data_ = nullptr;
+  size_type size_ = 0;
+};
+
+std::ostream& operator<<(std::ostream& o, string_view v);
+
+std::string& operator+=(std::string& s, string_view v);
+
+inline bool operator==(string_view l, string_view r) noexcept
+{
+  return l.compare(r) == 0;
+}
+
+inline bool operator!=(string_view l, string_view r) noexcept
+{
+  return l.compare(r) != 0;
+}
+
+inline bool operator<(string_view l, string_view r) noexcept
+{
+  return l.compare(r) < 0;
+}
+
+inline bool operator<=(string_view l, string_view r) noexcept
+{
+  return l.compare(r) <= 0;
+}
+
+inline bool operator>(string_view l, string_view r) noexcept
+{
+  return l.compare(r) > 0;
+}
+
+inline bool operator>=(string_view l, string_view r) noexcept
+{
+  return l.compare(r) >= 0;
+}
+}
+
+namespace std {
+
+template <>
+struct hash<cm::string_view>
+{
+  using argument_type = cm::string_view;
+  using result_type = size_t;
+  result_type operator()(argument_type const& s) const noexcept;
+};
+}
+
+#endif
+#endif
diff --git a/Utilities/std/cm/utility b/Utilities/std/cm/utility
new file mode 100644
index 0000000000..3acac4f69d
--- /dev/null
+++ b/Utilities/std/cm/utility
@@ -0,0 +1,34 @@
+// -*-c++-*-
+// vim: set ft=cpp:
+
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cm_utility
+#define cm_utility
+
+#if __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
+#  define CMake_HAVE_CXX_IN_PLACE
+#endif
+
+#include <utility> // IWYU pragma: export
+
+namespace cm {
+
+#if defined(CMake_HAVE_CXX_IN_PLACE)
+
+using std::in_place_t;
+using std::in_place;
+
+#else
+
+struct in_place_t
+{
+  explicit in_place_t() = default;
+};
+
+constexpr in_place_t in_place{};
+
+#endif
+}
+
+#endif