libstdc++: Constrain std::as_writable_bytes [PR101411]

The std::as_writable_bytes function should be constrained to only accept
writable spans. Currently it can be called but then gives an error in
the function body.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/101411
	* include/std/span (as_writable_bytes): Add requires-clause.
	* testsuite/23_containers/span/101411.cc: New test.

1 files changed, 15 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/23_containers/span/101411.cc b/libstdc++-v3/testsuite/23_containers/span/101411.cc
new file mode 100644
index 00000000000..05bdd3badbd
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/101411.cc
@@ -0,0 +1,15 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { xfail c++20 } }
+#include <span>
+
+// PR libstdc++/101411
+
+void f(std::span<const int> s)
+{
+  std::as_writable_bytes(s); // { dg-error "no matching function" }
+}
+
+void f1(std::span<const int, 1> s)
+{
+  std::as_writable_bytes(s); // { dg-error "no matching function" }
+}