gdb: use static_cast in gdb::checked_static_cast

This commit:

  commit 6fe4779ac4b1874c995345e3eabd89cb1a05fbdf
  Date:   Sat Feb 24 11:00:20 2024 +0100

      [gdb/build] Fix static cast of virtual base

addressed an issue where GDB would not compile in production mode due
to a use of gdb::checked_static_cast.  The problem was that we were
asking GDB to cast from a virtual base class to a sub-class, this
works fine when using dynamic_cast, but does not work with
static_cast.

The gdb::checked_static_cast actually uses dynamic_cast under the hood
in development mode in order to ensure that the cast is valid, while
in a production build we use static_cast as this is more efficient.

What this meant however, was that when gdb::checked_static_cast was
used to cast from a virtual base class, the dynamic_cast of a
non-production build worked fine, while the production build's
static_cast caused a build failure.

However, the gdb::checked_static_cast function already contains some
static_assert calls that are intended to catch any issues with invalid
type casting, the goal of these asserts was to prevent issues like
this: the build only failing in production mode.  Clearly the current
asserts are not enough.

I don't think there is a std::is_virtual_base type trait check, so
what I propose instead is that in non-production mode we also make use
of static_cast.  This will ensure that any errors that crop up in
production mode should also be revealed in non-production mode, and
should catch issues like this in the future.

There should be no user visible changes after this commit.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31399

Co-Authored-By: Simon Marchi <simon.marchi@polymtl.ca>

1 files changed, 4 insertions, 8 deletions

diff --git a/gdbsupport/gdb-checked-static-cast.h b/gdbsupport/gdb-checked-static-cast.h
index 227010e..24fa7a4 100644
--- a/gdbsupport/gdb-checked-static-cast.h
+++ b/gdbsupport/gdb-checked-static-cast.h
@@ -54,16 +54,12 @@ checked_static_cast (V *v)
 		 "types must be related");
 
 #ifdef DEVELOPMENT
-  if (v == nullptr)
-    return nullptr;
-
-  T result = dynamic_cast<T> (v);
-  gdb_assert (result != nullptr);
-#else
-  T result = static_cast<T> (v);
+  gdb_assert (v == nullptr || dynamic_cast<T> (v) != nullptr);
 #endif
 
-  return result;
+  /* If a base class of V is virtual then the dynamic_cast above will
+     succeed, but this static_cast will fail.  */
+  return static_cast<T> (v);
 }
 
 /* Same as the above, but to cast from a reference type to another.  */