Fix narrowing conversions in string_view types

	* include/experimental/string_view (basic_string_view::_S_compare):
	Use value-init so narrowing conversions are not ill-formed.
	* include/std/string_view (basic_string_view::_S_compare): Likewise.

From-SVN: r255321

libstdc++-v3/ChangeLog

 2017-12-01  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/experimental/string_view (basic_string_view::_S_compare):
+	Use value-init so narrowing conversions are not ill-formed.
+	* include/std/string_view (basic_string_view::_S_compare): Likewise.
+
 	* include/bits/basic_string.h (operator""s): Add pragmas to disable
 	-Wliteral-suffix warnings.
 	* include/experimental/string_view (operator""sv): Likewise.

libstdc++-v3/include/experimental/string_view

@@ -422,11 +422,11 @@ inline namespace fundamentals_v1
       static constexpr int
       _S_compare(size_type __n1, size_type __n2) noexcept
       {
-	return difference_type{__n1 - __n2} > std::numeric_limits<int>::max()
+	return difference_type(__n1 - __n2) > std::numeric_limits<int>::max()
 	     ? std::numeric_limits<int>::max()
-	     : difference_type{__n1 - __n2} < std::numeric_limits<int>::min()
+	     : difference_type(__n1 - __n2) < std::numeric_limits<int>::min()
 	     ? std::numeric_limits<int>::min()
-	     : static_cast<int>(difference_type{__n1 - __n2});
+	     : static_cast<int>(difference_type(__n1 - __n2));
       }
 
       size_t	    _M_len;

libstdc++-v3/include/std/string_view

@@ -408,7 +408,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static constexpr int
       _S_compare(size_type __n1, size_type __n2) noexcept
       {
-	const difference_type __diff{__n1 - __n2};
+	const difference_type __diff = __n1 - __n2;
 	if (__diff > std::numeric_limits<int>::max())
 	  return std::numeric_limits<int>::max();
 	if (__diff < std::numeric_limits<int>::min())