libstdc++: Make std::string_view(Range&&) constructor explicit

The P2499R0 paper was recently approved for C++23.

libstdc++-v3/ChangeLog:

	* include/std/string_view (basic_string_view(Range&&)): Add
	explicit as per P2499R0.
	* testsuite/21_strings/basic_string_view/cons/char/range_c++20.cc:
	Adjust implicit conversions. Check implicit conversions fail.
	* testsuite/21_strings/basic_string_view/cons/wchar_t/range_c++20.cc:
	Likewise.

libstdc++-v3/include/std/string_view

@@ -162,7 +162,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      })
 	  && (!requires { typename _DRange::traits_type; }
 	      || is_same_v<typename _DRange::traits_type, _Traits>)
-	constexpr
+	constexpr explicit
 	basic_string_view(_Range&& __r)
 	noexcept(noexcept(ranges::size(__r)) && noexcept(ranges::data(__r)))
 	: _M_len(ranges::size(__r)), _M_str(ranges::data(__r))

libstdc++-v3/testsuite/21_strings/basic_string_view/cons/char/range_c++20.cc

@@ -36,7 +36,7 @@ test01()
   };
 
   R r;
-  std::string_view s = r;
+  std::string_view s{r};
   VERIFY( s == r.str );
   VERIFY( s.data() == std::ranges::data(r) );
   VERIFY( s.size() == std::ranges::size(r) );
@@ -50,10 +50,15 @@ test01()
   static_assert( std::ranges::contiguous_range<R2> );
   static_assert( std::ranges::sized_range<R2> );
   R2 r2;
-  std::string_view s2 = r2; // uses conversion to string_view
+  std::string_view s2(r2); // uses conversion to string_view
   VERIFY( s2 == "Out of range" );
   VERIFY( std::string_view(const_cast<const R2&>(r2)) == s2 );
 
+  // And again using copy-initialization instead of direct-initialization.
+  std::string_view s2_implicit = r2; // uses conversion to string_view
+  VERIFY( s2_implicit == "Out of range" );
+  VERIFY( std::string_view(const_cast<const R2&>(r2)) == s2_implicit );
+
   struct R3 : R
   {
     using R::begin;
@@ -91,7 +96,7 @@ test01()
   static_assert( std::ranges::contiguous_range<R5> );
   static_assert( std::ranges::sized_range<R5> );
   R5 r5;
-  std::string_view s5 = r5; // Uses range constructor
+  std::string_view s5(r5); // Uses range constructor
   VERIFY( s5 == r5.str );
   s5 = std::string_view(std::move(r5)); // In C++20 this used conversion op.
   VERIFY( s5 == r5.str );	        // In C++23 it uses range constructor.
@@ -156,15 +161,30 @@ test04()
   };
 
   R r;
-  std::basic_string_view s = r; // Use deduction guide.
+  std::basic_string_view s(r); // Use deduction guide.
 
   static_assert( std::is_same_v<decltype(s), std::string_view> );
 }
 
+void
+test05()
+{
+  struct R
+  {
+    const char* begin() const { return nullptr; }
+    const char* end() const { return nullptr; }
+  };
+
+  // P2499R0 string_view range constructor should be explicit
+  // P2516R0 string_view is implicitly convertible from what?
+  static_assert( ! std::is_convertible_v<R, std::string_view> );
+}
+
 int main()
 {
   test01();
   test02();
   test03();
   test04();
+  test05();
 }

libstdc++-v3/testsuite/21_strings/basic_string_view/cons/wchar_t/range_c++20.cc

@@ -36,7 +36,7 @@ test01()
   };
 
   R r;
-  std::wstring_view s = r;
+  std::wstring_view s{r};
   VERIFY( s == r.str );
   VERIFY( s.data() == std::ranges::data(r) );
   VERIFY( s.size() == std::ranges::size(r) );
@@ -50,10 +50,15 @@ test01()
   static_assert( std::ranges::contiguous_range<R2> );
   static_assert( std::ranges::sized_range<R2> );
   R2 r2;
-  std::wstring_view s2 = r2; // uses conversion to wstring_view
+  std::wstring_view s2(r2); // uses conversion to wstring_view
   VERIFY( s2 == L"Out of range" );
   VERIFY( std::wstring_view(const_cast<const R2&>(r2)) == s2 );
 
+  // And again using copy-initialization instead of direct-initialization.
+  std::wstring_view s2_implicit = r2; // uses conversion to wstring_view
+  VERIFY( s2_implicit == L"Out of range" );
+  VERIFY( std::wstring_view(const_cast<const R2&>(r2)) == s2_implicit );
+
   struct R3 : R
   {
     using R::begin;
@@ -91,10 +96,10 @@ test01()
   static_assert( std::ranges::contiguous_range<R5> );
   static_assert( std::ranges::sized_range<R5> );
   R5 r5;
-  std::wstring_view s5 = r5; // Uses range constructor
+  std::wstring_view s5(r5); // Uses range constructor
   VERIFY( s5 == r5.str );
   s5 = std::wstring_view(std::move(r5)); // In C++20 this used conversion op.
-  VERIFY( s5 == r5.str );	        // In C++23 it uses range constructor.
+  VERIFY( s5 == r5.str );	         // In C++23 it uses range constructor.
 
   wchar_t arr[] = L"arrangement\0with\0nulls";
   std::wstring_view sa = arr; // Does not use range constructor
@@ -156,15 +161,30 @@ test04()
   };
 
   R r;
-  std::basic_string_view s = r; // Use deduction guide.
+  std::basic_string_view s(r); // Use deduction guide.
 
   static_assert( std::is_same_v<decltype(s), std::wstring_view> );
 }
 
+void
+test05()
+{
+  struct R
+  {
+    const wchar_t* begin() const { return nullptr; }
+    const wchar_t* end() const { return nullptr; }
+  };
+
+  // P2499R0 string_view range constructor should be explicit
+  // P2516R0 string_view is implicitly convertible from what?
+  static_assert( ! std::is_convertible_v<R, std::wstring_view> );
+}
+
 int main()
 {
   test01();
   test02();
   test03();
   test04();
+  test05();
 }