[PATCH] declare std::array members attribute const [PR101831]

[Martin Sebor <msebor@gmail.com>]

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Passing an uninitialized object to a function that takes its argument
by const reference is diagnosed by -Wmaybe-uninitialized because most
such functions read the argument.  The exceptions are functions that
don't access the object but instead use its address to compute
a result.  This includes a number of std::array member functions such
as std::array<N>::size() which returns the template argument N.  Such
functions may be candidates for attribute const which also avoids
the warning.  The attribute typically only benefits extern functions
that IPA cannot infer the property from, but in this case it helps
avoid the warning which runs very early on, even without optimization
or inlining.  The attached patch adds the attribute to a subset of
those member functions of std::array.  (It doesn't add it to const
member functions like cbegin() or front() that return a const_iterator
or const reference to the internal data.)

It might be possible to infer this property from inline functions
earlier on than during IPA and avoid having to annotate them explicitly.
That seems like an enhancement worth considering in the future.

Tested on x86_64-linux.

Martin

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Declare std::array members with attribute const [PR101831].

Resolves:
PR libstdc++/101831 - Spurious maybe-uninitialized warning on std::array::size

libstdc++-v3/ChangeLog:

	* include/std/array (begin): Declare const member function attribute
	const.
	(end, rbegin, rend, size, max_size, empty, data): Same.
	* testsuite/23_containers/array/capacity/empty.cc: Add test cases.
	* testsuite/23_containers/array/capacity/max_size.cc: Same.
	* testsuite/23_containers/array/capacity/size.cc: Same.
	* testsuite/23_containers/array/iterators/begin_end.cc: New test.

diff --git a/libstdc++-v3/include/std/array b/libstdc++-v3/include/std/array
index b4d8fc81a52..e45143fb329 100644
--- a/libstdc++-v3/include/std/array
+++ b/libstdc++-v3/include/std/array
@@ -127,7 +127,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { std::swap_ranges(begin(), end(), __other.begin()); }
 
       // Iterators.
-      [[__nodiscard__]]
+      [[__gnu__::__const__, __nodiscard__]]
       _GLIBCXX17_CONSTEXPR iterator
       begin() noexcept
       { return iterator(data()); }
@@ -137,7 +137,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       begin() const noexcept
       { return const_iterator(data()); }
 
-      [[__nodiscard__]]
+      [[__gnu__::__const__, __nodiscard__]]
       _GLIBCXX17_CONSTEXPR iterator
       end() noexcept
       { return iterator(data() + _Nm); }
@@ -147,7 +147,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       end() const noexcept
       { return const_iterator(data() + _Nm); }
 
-      [[__nodiscard__]]
+      [[__gnu__::__const__, __nodiscard__]]
       _GLIBCXX17_CONSTEXPR reverse_iterator
       rbegin() noexcept
       { return reverse_iterator(end()); }
@@ -157,7 +157,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       rbegin() const noexcept
       { return const_reverse_iterator(end()); }
 
-      [[__nodiscard__]]
+      [[__gnu__::__const__, __nodiscard__]]
       _GLIBCXX17_CONSTEXPR reverse_iterator
       rend() noexcept
       { return reverse_iterator(begin()); }
@@ -188,15 +188,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return const_reverse_iterator(begin()); }
 
       // Capacity.
-      [[__nodiscard__]]
+      [[__gnu__::__const__, __nodiscard__]]
       constexpr size_type
       size() const noexcept { return _Nm; }
 
-      [[__nodiscard__]]
+      [[__gnu__::__const__, __nodiscard__]]
       constexpr size_type
       max_size() const noexcept { return _Nm; }
 
-      [[__nodiscard__]]
+      [[__gnu__::__const__, __nodiscard__]]
       constexpr bool
       empty() const noexcept { return size() == 0; }
 
@@ -278,7 +278,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
  	           : _AT_Type::_S_ref(_M_elems, 0);
       }
 
-      [[__nodiscard__]]
+      [[__gnu__::__const__, __nodiscard__]]
       _GLIBCXX17_CONSTEXPR pointer
       data() noexcept
       { return _AT_Type::_S_ptr(_M_elems); }
diff --git a/libstdc++-v3/testsuite/23_containers/array/capacity/empty.cc b/libstdc++-v3/testsuite/23_containers/array/capacity/empty.cc
index 3f3f282ad9d..cecbae39e45 100644
--- a/libstdc++-v3/testsuite/23_containers/array/capacity/empty.cc
+++ b/libstdc++-v3/testsuite/23_containers/array/capacity/empty.cc
@@ -40,8 +40,26 @@ test01()
   }
 }
 
+#pragma GCC push_options
+#pragma GCC optimize "0"
+
+void
+test02()
+{
+  {
+    const size_t len = 3;
+    typedef std::array<int, len> array_type;
+    array_type a;
+
+    VERIFY( a.empty() == false );    // { dg-bogus "-Wmaybe-uninitialized"
+  }
+}
+
+#pragma GCC pop_options
+
 int main()
 {
   test01();
+  test02();
   return 0;
 }
diff --git a/libstdc++-v3/testsuite/23_containers/array/capacity/max_size.cc b/libstdc++-v3/testsuite/23_containers/array/capacity/max_size.cc
index 0e000258530..4629316161d 100644
--- a/libstdc++-v3/testsuite/23_containers/array/capacity/max_size.cc
+++ b/libstdc++-v3/testsuite/23_containers/array/capacity/max_size.cc
@@ -40,8 +40,26 @@ test01()
   }
 }
 
+#pragma GCC push_options
+#pragma GCC optimize "0"
+
+void
+test02()
+{
+  {
+    const size_t len = 3;
+    typedef std::array<int, len> array_type;
+    array_type a;
+
+    VERIFY( a.max_size() == len );  // { dg-bogus "-Wmaybe-uninitialized"
+  }
+}
+
+#pragma GCC pop_options
+
 int main()
 {
   test01();
+  test02();
   return 0;
 }
diff --git a/libstdc++-v3/testsuite/23_containers/array/capacity/size.cc b/libstdc++-v3/testsuite/23_containers/array/capacity/size.cc
index 3e4aa7143dc..dddd909a0ac 100644
--- a/libstdc++-v3/testsuite/23_containers/array/capacity/size.cc
+++ b/libstdc++-v3/testsuite/23_containers/array/capacity/size.cc
@@ -40,8 +40,26 @@ test01()
   }
 }
 
+#pragma GCC push_options
+#pragma GCC optimize "0"
+
+void
+test02()
+{
+  {
+    const size_t len = 3;
+    typedef std::array<int, len> array_type;
+    array_type a;
+
+    VERIFY( a.size() == len );      // { dg-bogus "-Wmaybe-uninitialized"
+  }
+}
+
+#pragma GCC pop_options
+
 int main()
 {
   test01();
+  test02();
   return 0;
 }
diff --git a/libstdc++-v3/testsuite/23_containers/array/iterators/begin_end.cc b/libstdc++-v3/testsuite/23_containers/array/iterators/begin_end.cc
new file mode 100644
index 00000000000..b7743adf3c9
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/array/iterators/begin_end.cc
@@ -0,0 +1,56 @@
+// { dg-do compile { target c++11 } }
+//
+// Copyright (C) 2011-2022 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#include <array>
+
+#pragma GCC push_options
+#pragma GCC optimize "0"
+
+extern void
+sink (const void*, ...);
+
+void
+test01()
+{
+  {
+    const std::size_t len = 1;
+    typedef std::array<int, len> array_type;
+    typedef array_type::iterator iterator;;
+    array_type a;
+
+    iterator b = a.begin();           // { dg-bogus "-Wmaybe-uninitialized" }
+    iterator e = a.end();             // { dg-bogus "-Wmaybe-uninitialized" }
+
+    sink(&b, &e);
+  }
+
+  {
+    const std::size_t len = 3;
+    typedef std::array<int, len> array_type;
+    typedef array_type::reverse_iterator reverse_iterator;
+    array_type a;
+
+    reverse_iterator b = a.rbegin();  // { dg-bogus "-Wmaybe-uninitialized" }
+    reverse_iterator e = a.rend();    // { dg-bogus "-Wmaybe-uninitialized" }
+
+    sink(&b, &e);
+  }
+}
+
+#pragma GCC pop_options