Fix basic_string_view typedefs and enforce preconditions

The basic_string_view::pointer and basic_string_view::reference typedefs
are supposed to refer to the non-const value type.

In previous standards having traits_type::char_type different to
value_type was simply undefined, but in the C++2a draft it's ill-formed,
as changed by P1148R0. For std::basic_string and iostreams we might
want to only enforce this conditionally for __cplusplus > 201703L but
for std::basic_string_view we don't have backwards compatibility
concerns. Also add assertions to verify the _CharT argument is a
"char-like" type (non-array, trivial, standard layout type).

Also remove the non-standard basic_string_view::_M_check and
basic_string_view::_M_limit member functions, replacing them with
non-member functions that will still exist even if basic_string_view is
specialized by the program.

	* include/experimental/string_view (basic_string_view::pointer)
	(basic_string_view::reference): Fix to refer to non-const value_type.
	* include/bits/basic_string.h (basic_string): Use __sv_check and
	__sv_limit instead of basic_string_view::_M_check and
	basic_string_view::_M_limit.
	* include/std/string_view (__sv_check, __sv_limit): New
	helper functions to replace basic_string_view::_M_check and
	basic_string_view::_M_limit.
	(basic_string_view): Add static assertions to enforce ill-formed
	requirement for traits_type::char_type from P1148R0, and to enforce
	required properties of char-like types.
	(basic_string_view::pointer, basic_string_view::reference): Fix to
	refer to non-const value_type.
	(basic_string_view::operator[], basic_string_view::at)
	(basic_string_view::front, basic_string_view::back)
	(basic_string_view::data): Use const_reference and const_pointer
	typedefs for return types.
	(basic_string_view::_M_check, basic_string_view::_M_limit): Remove.
	(hash<wstring_view>): Fix argument_type typedef.
	* testsuite/21_strings/basic_string_view/modifiers/remove_prefix/
	char/1.cc: Fix expected return type of basic_string_view::data().
	* testsuite/21_strings/basic_string_view/modifiers/remove_prefix/
	wchar_t/1.cc: Likewise.
	* testsuite/21_strings/basic_string_view/modifiers/remove_suffix/
	char/1.cc: Likewise.
	* testsuite/21_strings/basic_string_view/modifiers/remove_suffix/
	wchar_t/1.cc: Likewise.
	* testsuite/21_strings/basic_string_view/requirements/traits_neg.cc:
	New test.
	* testsuite/21_strings/basic_string_view/requirements/typedefs.cc:
	Check reference and pointer typedefs.
	* testsuite/experimental/string_view/requirements/typedefs.cc:
	Likewise.
	* testsuite/experimental/string_view/modifiers/remove_prefix/char/1.cc:
	Fix expected return type of basic_string_view::data().
	* testsuite/experimental/string_view/modifiers/remove_prefix/wchar_t/
	1.cc: Likewise.
	* testsuite/experimental/string_view/modifiers/remove_suffix/char/1.cc:
	Likewise.
	* testsuite/experimental/string_view/modifiers/remove_suffix/wchar_t/
	1.cc: Likewise.

From-SVN: r270548

1 files changed, 75 insertions, 68 deletions

diff --git a/libstdc++-v3/include/std/string_view b/libstdc++-v3/include/std/string_view
index 844cfb1..42822cc 100644
--- a/libstdc++-v3/include/std/string_view
+++ b/libstdc++-v3/include/std/string_view
@@ -49,6 +49,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #define __cpp_lib_string_view 201603
 
+  // Helper for basic_string and basic_string_view members.
+  constexpr size_t
+  __sv_check(size_t __size, size_t __pos, const char* __s)
+  {
+    if (__pos > __size)
+      __throw_out_of_range_fmt(__N("%s: __pos (which is %zu) > __size "
+				   "(which is %zu)"), __s, __pos, __size);
+    return __pos;
+  }
+
+  // Helper for basic_string members.
+  // NB: __sv_limit doesn't check for a bad __pos value.
+  constexpr size_t
+  __sv_limit(size_t __size, size_t __pos, size_t __off) noexcept
+  {
+   const bool __testoff =  __off < __size - __pos;
+   return __testoff ? __off : __size - __pos;
+  }
+
   /**
    *  @class basic_string_view <string_view>
    *  @brief  A non-owning reference to a string.
@@ -70,24 +89,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _CharT, typename _Traits = std::char_traits<_CharT>>
     class basic_string_view
     {
+      static_assert(!is_array_v<_CharT>);
+      static_assert(is_trivial_v<_CharT> && is_standard_layout_v<_CharT>);
+      static_assert(is_same_v<_CharT, typename _Traits::char_type>);
+
     public:
 
       // types
-      using traits_type = _Traits;
-      using value_type = _CharT;
-      using pointer = const _CharT*;
-      using const_pointer = const _CharT*;
-      using reference = const _CharT&;
-      using const_reference = const _CharT&;
-      using const_iterator = const _CharT*;
-      using iterator = const_iterator;
+      using traits_type		= _Traits;
+      using value_type		= _CharT;
+      using pointer		= value_type*;
+      using const_pointer	= const value_type*;
+      using reference		= value_type&;
+      using const_reference	= const value_type&;
+      using const_iterator	= const value_type*;
+      using iterator		= const_iterator;
       using const_reverse_iterator = std::reverse_iterator<const_iterator>;
-      using reverse_iterator = const_reverse_iterator;
-      using size_type = size_t;
-      using difference_type = ptrdiff_t;
+      using reverse_iterator	= const_reverse_iterator;
+      using size_type		= size_t;
+      using difference_type	= ptrdiff_t;
       static constexpr size_type npos = size_type(-1);
 
-      // [string.view.cons], construct/copy
+      // [string.view.cons], construction and assignment
 
       constexpr
       basic_string_view() noexcept
@@ -110,7 +133,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr basic_string_view&
       operator=(const basic_string_view&) noexcept = default;
 
-      // [string.view.iterators], iterators
+      // [string.view.iterators], iterator support
 
       constexpr const_iterator
       begin() const noexcept
@@ -167,7 +190,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       // [string.view.access], element access
 
-      constexpr const _CharT&
+      constexpr const_reference
       operator[](size_type __pos) const noexcept
       {
 	// TODO: Assert to restore in a way compatible with the constexpr.
@@ -175,7 +198,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *(this->_M_str + __pos);
       }
 
-      constexpr const _CharT&
+      constexpr const_reference
       at(size_type __pos) const
       {
 	if (__pos >= _M_len)
@@ -185,7 +208,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *(this->_M_str + __pos);
       }
 
-      constexpr const _CharT&
+      constexpr const_reference
       front() const noexcept
       {
 	// TODO: Assert to restore in a way compatible with the constexpr.
@@ -193,7 +216,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *this->_M_str;
       }
 
-      constexpr const _CharT&
+      constexpr const_reference
       back() const noexcept
       {
 	// TODO: Assert to restore in a way compatible with the constexpr.
@@ -201,7 +224,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	return *(this->_M_str + this->_M_len - 1);
       }
 
-      constexpr const _CharT*
+      constexpr const_pointer
       data() const noexcept
       { return this->_M_str; }
 
@@ -233,7 +256,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       copy(_CharT* __str, size_type __n, size_type __pos = 0) const
       {
 	__glibcxx_requires_string_len(__str, __n);
-	__pos = _M_check(__pos, "basic_string_view::copy");
+	__pos = std::__sv_check(size(), __pos, "basic_string_view::copy");
 	const size_type __rlen = std::min(__n, _M_len - __pos);
 	// _GLIBCXX_RESOLVE_LIB_DEFECTS
 	// 2777. basic_string_view::copy should use char_traits::copy
@@ -244,7 +267,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr basic_string_view
       substr(size_type __pos = 0, size_type __n = npos) const noexcept(false)
       {
-	__pos = _M_check(__pos, "basic_string_view::substr");
+	__pos = std::__sv_check(size(), __pos, "basic_string_view::substr");
 	const size_type __rlen = std::min(__n, _M_len - __pos);
 	return basic_string_view{_M_str + __pos, __rlen};
       }
@@ -286,6 +309,37 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		   .compare(basic_string_view(__str, __n2));
       }
 
+#if __cplusplus > 201703L
+      constexpr bool
+      starts_with(basic_string_view __x) const noexcept
+      { return this->substr(0, __x.size()) == __x; }
+
+      constexpr bool
+      starts_with(_CharT __x) const noexcept
+      { return !this->empty() && traits_type::eq(this->front(), __x); }
+
+      constexpr bool
+      starts_with(const _CharT* __x) const noexcept
+      { return this->starts_with(basic_string_view(__x)); }
+
+      constexpr bool
+      ends_with(basic_string_view __x) const noexcept
+      {
+	return this->size() >= __x.size()
+	    && this->compare(this->size() - __x.size(), npos, __x) == 0;
+      }
+
+      constexpr bool
+      ends_with(_CharT __x) const noexcept
+      { return !this->empty() && traits_type::eq(this->back(), __x); }
+
+      constexpr bool
+      ends_with(const _CharT* __x) const noexcept
+      { return this->ends_with(basic_string_view(__x)); }
+#endif // C++20
+
+      // [string.view.find], searching
+
       constexpr size_type
       find(basic_string_view __str, size_type __pos = 0) const noexcept
       { return this->find(__str._M_str, __pos, __str._M_len); }
@@ -386,53 +440,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				      traits_type::length(__str));
       }
 
-#if __cplusplus > 201703L
-      constexpr bool
-      starts_with(basic_string_view __x) const noexcept
-      { return this->substr(0, __x.size()) == __x; }
-
-      constexpr bool
-      starts_with(_CharT __x) const noexcept
-      { return !this->empty() && traits_type::eq(this->front(), __x); }
-
-      constexpr bool
-      starts_with(const _CharT* __x) const noexcept
-      { return this->starts_with(basic_string_view(__x)); }
-
-      constexpr bool
-      ends_with(basic_string_view __x) const noexcept
-      {
-	return this->size() >= __x.size()
-	    && this->compare(this->size() - __x.size(), npos, __x) == 0;
-      }
-
-      constexpr bool
-      ends_with(_CharT __x) const noexcept
-      { return !this->empty() && traits_type::eq(this->back(), __x); }
-
-      constexpr bool
-      ends_with(const _CharT* __x) const noexcept
-      { return this->ends_with(basic_string_view(__x)); }
-#endif // C++20
-
-      constexpr size_type
-      _M_check(size_type __pos, const char* __s) const noexcept(false)
-      {
-	if (__pos > this->size())
-	  __throw_out_of_range_fmt(__N("%s: __pos (which is %zu) > "
-				       "this->size() (which is %zu)"),
-				   __s, __pos, this->size());
-	return __pos;
-      }
-
-      // NB: _M_limit doesn't check for a bad __pos value.
-      constexpr size_type
-      _M_limit(size_type __pos, size_type __off) const noexcept
-      {
-	const bool __testoff =  __off < this->size() - __pos;
-	return __testoff ? __off : this->size() - __pos;
-      }
-      
     private:
 
       static constexpr int
@@ -610,7 +617,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #ifdef _GLIBCXX_USE_WCHAR_T
   template<>
     struct hash<wstring_view>
-    : public __hash_base<size_t, wstring>
+    : public __hash_base<size_t, wstring_view>
     {
       size_t
       operator()(const wstring_view& __s) const noexcept