Handle pointers and references correctly in DAP

A user pointed out that the current DAP variable code does not let the
client deference a pointer.  Oops!

Fixing this oversight is simple enough -- adding a new no-op
pretty-printer for pointers and references is quite simple.

However, doing this naive caused a regession in scopes.exp, which
expected there to be no children of a 'const char *' variable.  This
problem was fixed by the preceding patches in the series, which ensure
that a C type of this kind is recognized as a string.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30821

gdb/python/lib/gdb/printing.py

@@ -280,6 +280,20 @@ class NoOpScalarPrinter:
         return self.value.format_string(raw=True)
 
 
+class NoOpPointerReferencePrinter:
+    """A no-op pretty printer that wraps a pointer or reference."""
+
+    def __init__(self, value):
+        self.value = value
+        self.num_children = 1
+
+    def to_string(self):
+        return self.value.format_string(deref_refs=False)
+
+    def children(self):
+        yield "value", self.value.referenced_value()
+
+
 class NoOpArrayPrinter:
     """A no-op pretty printer that wraps an array value."""
 
@@ -354,6 +368,8 @@ def make_visualizer(value):
             result = gdb.printing.NoOpArrayPrinter(ty, value)
         elif ty.code in (gdb.TYPE_CODE_STRUCT, gdb.TYPE_CODE_UNION):
             result = gdb.printing.NoOpStructPrinter(ty, value)
+        elif ty.code in (gdb.TYPE_CODE_PTR, gdb.TYPE_CODE_REF, gdb.TYPE_CODE_RVALUE_REF):
+            result = NoOpPointerReferencePrinter(value)
         else:
             result = gdb.printing.NoOpScalarPrinter(value)
     return result

gdb/testsuite/gdb.dap/ptrref.cc

@@ -0,0 +1,34 @@
+/* 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/>.  */
+
+void
+func ()
+{
+  int value = 23;
+
+  int *ptr = &value;
+  int &ref = value;
+
+  return;			/* BREAK */
+}
+
+int
+main (int argc, char *argv[])
+{
+  func ();
+  return 0;
+}

gdb/testsuite/gdb.dap/ptrref.exp

@@ -0,0 +1,103 @@
+# 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/>.
+
+require allow_dap_tests
+
+load_lib dap-support.exp
+
+standard_testfile .cc
+
+if {[build_executable ${testfile}.exp $testfile $srcfile {debug c++}] == -1} {
+    return
+}
+
+if {[dap_launch $testfile] == ""} {
+    return
+}
+
+set line [gdb_get_line_number "BREAK"]
+set obj [dap_check_request_and_response "set breakpoint by line number" \
+	     setBreakpoints \
+	     [format {o source [o path [%s]] breakpoints [a [o line [i %d]]]} \
+		  [list s $srcfile] $line]]
+set line_bpno [dap_get_breakpoint_number $obj]
+
+dap_check_request_and_response "start inferior" configurationDone
+dap_wait_for_event_and_check "inferior started" thread "body reason" started
+
+dap_wait_for_event_and_check "stopped at line breakpoint" stopped \
+    "body reason" breakpoint \
+    "body hitBreakpointIds" $line_bpno
+
+set bt [lindex [dap_check_request_and_response "backtrace" stackTrace \
+		    {o threadId [i 1]}] \
+	    0]
+set frame_id [dict get [lindex [dict get $bt body stackFrames] 0] id]
+
+set scopes [dap_check_request_and_response "get scopes" scopes \
+		[format {o frameId [i %d]} $frame_id]]
+set scopes [dict get [lindex $scopes 0] body scopes]
+
+gdb_assert {[llength $scopes] == 2} "two scopes"
+
+lassign $scopes scope reg_scope
+gdb_assert {[dict get $scope name] == "Locals"} "scope is locals"
+
+gdb_assert {[dict get $scope namedVariables] == 3} "three vars in scope"
+
+set num [dict get $scope variablesReference]
+set refs [lindex [dap_check_request_and_response "fetch variables" \
+		      "variables" \
+		      [format {o variablesReference [i %d] count [i 3]} \
+			   $num]] \
+	      0]
+
+proc fetch_pointer {name var} {
+    set num [dict get $var variablesReference]
+    set refs [lindex [dap_check_request_and_response "fetch children of $name" \
+			  "variables" \
+			  [format {o variablesReference [i %d] count [i 1]} \
+			       $num]] \
+		  0]
+    set var [lindex [dict get $refs body variables] 0]
+    gdb_assert {[dict get $var value] == 23} \
+	"value of child of $name"
+    gdb_assert {[dict get $var variablesReference] == 0} \
+	"no children for child of $name"
+}
+
+foreach var [dict get $refs body variables] {
+    set name [dict get $var name]
+
+    switch $name {
+	"value" {
+	    gdb_assert {[dict get $var value] == "23"} "check value of value"
+	    gdb_assert {[dict get $var variablesReference] == 0} \
+		"$name does not have child"
+	}
+	"ptr" -
+	"ref" {
+	    gdb_assert {[dict get $var variablesReference] != 0} \
+		"$name has children"
+	    # The implementation somewhat arbitrarily chooses "named"
+	    # children here.
+	    gdb_assert {[dict get $var namedVariables] == 1} \
+		"$name has exactly one child"
+	    fetch_pointer $name $var
+	}
+    }
+}
+
+dap_shutdown