libstdc++: Fix bug in std::span test

The previous commit fixed the std::span constructors from const arrays,
revealing a bug in this test.

	* testsuite/23_containers/span/lwg3255.cc: Fix test. Constructing a
	span of non-const elements should not be possible from a const array
	or an array of const elements.

From-SVN: r279001

libstdc++-v3/ChangeLog

+2019-12-05  Jonathan Wakely  <jwakely@redhat.com>
+
+	* testsuite/23_containers/span/lwg3255.cc: Fix test. Constructing a
+	span of non-const elements should not be possible from a const array
+	or an array of const elements.
+
 2019-12-05  JeanHeyd "ThePhD" Meneide  <phdofthehouse@gmail.com>
 
 	Implement P1872R0 and P1394R0 for std::span

libstdc++-v3/testsuite/23_containers/span/lwg3255.cc

@@ -39,7 +39,6 @@ static_assert( is_constructible_v<span<int, 1>, array<int, 1>&> );
 static_assert( is_constructible_v<span<const int, 1>, array<int, 1>&> );
 static_assert( is_constructible_v<span<const int, 1>, array<const int, 1>&> );
 
-static_assert( is_constructible_v<span<int, 1>, const array<int, 1>&> );
 static_assert( is_constructible_v<span<const int, 1>, const array<int, 1>&> );
 static_assert( is_constructible_v<span<const int, 1>, const array<const int, 1>&> );
 
@@ -63,6 +62,12 @@ static_assert( is_constructible_v<span<int>, array<int, 2>&> );
 static_assert( is_constructible_v<span<const int>, array<int, 2>&> );
 static_assert( is_constructible_v<span<const int>, array<const int, 2>&> );
 
-static_assert( is_constructible_v<span<int>, const array<int, 2>&> );
 static_assert( is_constructible_v<span<const int>, const array<int, 2>&> );
 static_assert( is_constructible_v<span<const int>, const array<const int, 2>&> );
+
+static_assert( ! is_constructible_v<span<int, 1>, array<const int, 1>&> );
+static_assert( ! is_constructible_v<span<int, 1>, const array<int, 1>&> );
+static_assert( ! is_constructible_v<span<int, 1>, const array<const int, 1>&> );
+static_assert( ! is_constructible_v<span<int>, array<const int, 2>&> );
+static_assert( ! is_constructible_v<span<int>, const array<int, 2>&> );
+static_assert( ! is_constructible_v<span<int>, const array<const int, 2>&> );