[PATCH 2/2] Improve gdb::array_view ctor from contiguous containers

[Lancelot SIX <lsix@lancelotsix.com>]

While reading the interface of gdb::array_view, I realized that the
constructor that builds an array_view on top of a contiguous container
(such as std::vector, std::array or even gdb::array_view) can be
miss-used.

Lets consider the following code sample:

	struct Parent
	{
	  Parent (int a): a { a } {}
	  int a;
	};

	struct Child : public Parent
	{
	  Child (int a, int b): Parent { a }, b { b } {}
	  int b;
	};

	std::ostream &operator<< (std::ostream& os, const Parent & p)
	{ os << "Parent {a=" << p.a << "}"; return os; }
	std::ostream &operator<< (std::ostream& os, const Child & p)
	{ os << "Child {a=" << p.a << ", b=" << p.b << "}"; return os; }

	template <typename T>
	void print_view (const gdb::array_view<const T> &p)
	{
	  std::for_each (p.begin (), p.end (), [](const T &e) { std::cout << e << '\n'; });
	}

Then with the current version of the interface we can write something
like:

	const std::array<Child, 3> elts = {
	  Child {1, 2},
	  Child {3, 4},
	  Child {5, 6}
	};
	print_view<Parent> (elts);

This compiles fine and produces the following output:

	Parent {a=1}
	Parent {a=2}
	Parent {a=3}

This is obviously wrong.  There is nowhere in memory a Parent-like
object for which the A member is 2.  This call to print_all<Parent>
should not compile at all (a call to print_all<Child> instead would
however be fine).

This comes down to the fact that the constructor used in this example
only requires that a Child* is convertible into a Parent*.  This
requirement is valid necessary but not sufficient.

The other constructors of a array_view<T> from data of type U have
stricter requirements on what U should be.  It should satisfy that a U*
can be converted to a T*, but they also require that (decayed) T and
(decayed) U are the same.

This patch proposes to change the constraints on the gdb::array_view
constructor that accepts a Container<U>.  It makes it so the
requirements on what U should be are now the same as for the other
constructors.

Applying this change required minimum adjustment in GDB codebase (two
'const' addition), which are also included in this patch.

Tested by rebuilding using GCC-10.3.0 on GNU/Linux.
---
 gdb/dwarf2/read.c       | 2 +-
 gdb/maint.c             | 2 +-
 gdbsupport/array-view.h | 7 ++++---
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index cbd9a3012eb..0b82f1bbad4 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -14934,7 +14934,7 @@ add_variant_property (struct field_info *fip, struct type *type,
     offset_map[fip->fields[i].offset] = i;
 
   struct objfile *objfile = cu->per_objfile->objfile;
-  gdb::array_view<variant_part> parts
+  gdb::array_view<const variant_part> parts
     = create_variant_parts (&objfile->objfile_obstack, offset_map, fip,
 			    fip->variant_parts);
 
diff --git a/gdb/maint.c b/gdb/maint.c
index 8aae53bdd65..a5e21f458ad 100644
--- a/gdb/maint.c
+++ b/gdb/maint.c
@@ -1128,7 +1128,7 @@ maintenance_selftest (const char *args, int from_tty)
 {
 #if GDB_SELF_TEST
   bool verbose = args != nullptr && check_for_argument (&args, "-verbose");
-  gdb_argv argv (args);
+  const gdb_argv argv (args);
   selftests::run_tests (argv.as_array_view (), verbose);
 #else
   printf_filtered (_("\
diff --git a/gdbsupport/array-view.h b/gdbsupport/array-view.h
index c41fd620f83..565ad988974 100644
--- a/gdbsupport/array-view.h
+++ b/gdbsupport/array-view.h
@@ -139,9 +139,10 @@ class array_view
   template<typename Container,
 	   typename = Requires<gdb::Not<IsDecayedT<Container>>>,
 	   typename
-	     = Requires<std::is_convertible
-			<decltype (std::declval<Container> ().data ()),
-			 T *>>,
+	     = Requires<DecayedConvertible
+			<typename std::remove_pointer
+			 <decltype (std::declval<Container> ().data ())
+			 >::type>>,
 	   typename
 	     = Requires<std::is_convertible
 			<decltype (std::declval<Container> ().size ()),
-- 
2.33.0