Use pretty printers for struct member stubs

PR29079 shows that pretty printers can be used for an incomplete
type (stub), but only when printing it directly, not if it's
part of another struct:
```
(gdb) p s
$1 = {pp m_i = 5}
(gdb) p s2
$2 = {m_s = <incomplete type>, m_l = 20}
```

The reason is simply that in common_val_print the check for stubs
is before any pretty printer is tried.
It works if the pretty printer is tried before the stub check:
```
(gdb) p s
$1 = {pp m_i = 5}
(gdb) p s2
$2 = {m_s = {pp m_i = 10}, m_l = 20}
```

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29079
Approved-By: Tom Tromey <tom@tromey.com>

gdb/testsuite/gdb.python/py-prettyprint-stub-2.cc

@@ -0,0 +1,26 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2023 Free Software Foundation, Inc.
+
+   This program 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 of the License, or
+   (at your option) any later version.
+
+   This program 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 program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "py-prettyprint-stub.h"
+
+S::S (int i) : m_i (i)
+{
+}
+
+S::~S ()
+{
+}

gdb/testsuite/gdb.python/py-prettyprint-stub.cc

@@ -0,0 +1,36 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2023 Free Software Foundation, Inc.
+
+   This program 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 of the License, or
+   (at your option) any later version.
+
+   This program 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 program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "py-prettyprint-stub.h"
+
+struct S2
+{
+  S2 (int i, long l) : m_s (i), m_l (l)
+  {
+  }
+
+  S m_s;
+  long m_l;
+};
+
+int main ()
+{
+  S s (5);
+  S2 s2 (10, 20);
+
+  return 0; /* Break here.  */
+}

gdb/testsuite/gdb.python/py-prettyprint-stub.exp

@@ -0,0 +1,59 @@
+# Copyright (C) 2023 Free Software Foundation, Inc.
+
+# This program 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 of the License, or
+# (at your option) any later version.
+#
+# This program 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 program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of the GDB testsuite.
+# It tests Python-based pretty-printing of stubs.
+
+load_lib gdb-python.exp
+
+require allow_python_tests
+
+standard_testfile .cc py-prettyprint-stub-2.cc
+
+set srcfiles [list $srcfile $srcfile2]
+
+if { [build_executable_from_specs \
+	  "failed to prepare" \
+	  $testfile {c++} \
+	  $srcfile {c++ debug} \
+	  $srcfile2 {c++}] == -1 } {
+    return -1
+}
+
+# Start with a fresh gdb.
+clean_restart $testfile
+
+if {![runto_main]} {
+    return
+}
+
+set remote_python_file [gdb_remote_download host \
+			${srcdir}/${subdir}/${testfile}.py]
+
+gdb_breakpoint [gdb_get_line_number "Break here."]
+gdb_continue_to_breakpoint "Break here" ".*Break here.*"
+
+# First test without pretty printer.
+gdb_test "print s" " = <incomplete type>"
+gdb_test "print s2" " = {m_s = <incomplete type>, m_l = 20}"
+
+# Load pretty printer.
+gdb_test_no_output "source ${remote_python_file}" "load python file"
+
+# Test with pretty printer.
+with_test_prefix pp {
+    gdb_test "print s" " = {pp m_i = 5}"
+    gdb_test "print s2" " = {m_s = {pp m_i = 10}, m_l = 20}"
+}

gdb/testsuite/gdb.python/py-prettyprint-stub.h

@@ -0,0 +1,24 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2023 Free Software Foundation, Inc.
+
+   This program 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 of the License, or
+   (at your option) any later version.
+
+   This program 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 program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+struct S
+{
+  S (int);
+  virtual ~S ();
+
+  int m_i;
+};

gdb/testsuite/gdb.python/py-prettyprint-stub.py

@@ -0,0 +1,38 @@
+# Copyright (C) 2023 Free Software Foundation, Inc.
+
+# This program 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 of the License, or
+# (at your option) any later version.
+#
+# This program 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 program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of the GDB testsuite.
+# It tests Python-based pretty-printing of stubs.
+
+
+class SPrinter:
+    def __init__(self, val):
+        self.val = val
+
+    def to_string(self):
+        char_ptr = gdb.lookup_type("char").pointer()
+        int_ptr = gdb.lookup_type("int").pointer()
+        # m_i should be after the vtable, which has the size of a pointer
+        i = (
+            (self.val.address.cast(char_ptr) + int_ptr.sizeof)
+            .cast(int_ptr)
+            .dereference()
+        )
+        return "{pp m_i = %d}" % int(i)
+
+
+pp = gdb.printing.RegexpCollectionPrettyPrinter("S")
+pp.add_printer("S", "^S$", SPrinter)
+gdb.printing.register_pretty_printer(gdb.current_objfile(), pp, True)

gdb/valprint.c

@@ -1054,16 +1054,6 @@ common_val_print (struct value *value, struct ui_file *stream, int recurse,
 
   QUIT;
 
-  /* Ensure that the type is complete and not just a stub.  If the type is
-     only a stub and we can't find and substitute its complete type, then
-     print appropriate string and return.  */
-
-  if (real_type->is_stub ())
-    {
-      fprintf_styled (stream, metadata_style.style (), _("<incomplete type>"));
-      return;
-    }
-
   if (!valprint_check_validity (stream, real_type, 0, value))
     return;
 
@@ -1074,6 +1064,16 @@ common_val_print (struct value *value, struct ui_file *stream, int recurse,
 	return;
     }
 
+  /* Ensure that the type is complete and not just a stub.  If the type is
+     only a stub and we can't find and substitute its complete type, then
+     print appropriate string and return.  */
+
+  if (real_type->is_stub ())
+    {
+      fprintf_styled (stream, metadata_style.style (), _("<incomplete type>"));
+      return;
+    }
+
   /* Handle summary mode.  If the value is a scalar, print it;
      otherwise, print an ellipsis.  */
   if (options->summary && !val_print_scalar_type_p (type))