[gcc r11-9588] libstdc++: missing constexpr for __[nm]iter_base [PR102358]

[Patrick Palka]

https://gcc.gnu.org/g:973e0bc1542dd5efc501fef8653d47c9b00adf97

commit r11-9588-g973e0bc1542dd5efc501fef8653d47c9b00adf97
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Oct 21 12:13:35 2021 -0400

    libstdc++: missing constexpr for __[nm]iter_base [PR102358]
    
            PR libstdc++/102358
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/stl_iterator.h (__niter_base): Make constexpr
            for C++20.
            (__miter_base): Likewise.
            * testsuite/25_algorithms/move/constexpr.cc: New test.
    
    (cherry picked from commit 5f7976f65b45c457b57bfc2c55ec845771e0d3c2)

Diff:
---
 libstdc++-v3/include/bits/stl_iterator.h              |  2 ++
 .../testsuite/25_algorithms/move/constexpr.cc         | 19 +++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 549bc26dee5..2b7efcbf3c3 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -2338,6 +2338,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   /// @} group iterators
 
   template<typename _Iterator>
+    _GLIBCXX20_CONSTEXPR
     auto
     __niter_base(move_iterator<_Iterator> __it)
     -> decltype(make_move_iterator(__niter_base(__it.base())))
@@ -2351,6 +2352,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     };
 
   template<typename _Iterator>
+    _GLIBCXX20_CONSTEXPR
     auto
     __miter_base(move_iterator<_Iterator> __it)
     -> decltype(__miter_base(__it.base()))
diff --git a/libstdc++-v3/testsuite/25_algorithms/move/constexpr.cc b/libstdc++-v3/testsuite/25_algorithms/move/constexpr.cc
new file mode 100644
index 00000000000..773c55cfb50
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/move/constexpr.cc
@@ -0,0 +1,19 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+
+#include <algorithm>
+#include <span>
+
+constexpr bool
+test01()
+{
+  // PR libstdc++/102358
+  int x[2] = {1,2}, y[2];
+  std::span in(x), out(y);
+  std::move(std::move_iterator(in.begin()), std::move_iterator(in.end()),
+	    out.begin());
+  return std::equal(std::move_iterator(in.begin()), std::move_iterator(in.end()),
+		    std::move_iterator(out.begin()));
+}
+
+static_assert(test01());