Fix Ada overloading with 'null'

Currently, the Ada expression parser treats 'null' as an integer 0.
However, this causes overloading to fail in certain cases.

This patch changes the Ada expression parser to use a special type for
'null'.  I chose pointer-to-int0, because I think that's not likely to
be needed for any other Ada expression.  Note this works because a
"mod 1" type has an underlying non-zero byte size; the test includes a
check for this.

The output is changed so that "print null", by default, shows "null".
And, ada_type_match is changed both to recognize the special null type
and to remove a bit of weird code related to how pointers are treated
for overload type matching.

Tested on x86-64 Fedora 32.  Because this only touches Ada, and Joel
already approved it internally at AdaCore, I am checking it in.

gdb/ChangeLog
2021-04-28  Tom Tromey  <tromey@adacore.com>

	* ada-exp.y (primary): Use new type for null pointer.
	* ada-lang.c (ada_type_match): Remove "may_deref"
	parameter.  Handle null pointer.
	(ada_args_match): Update.
	* ada-valprint.c (ada_value_print_ptr, ada_value_print):
	Handle null pointer.

gdb/testsuite/ChangeLog
2021-04-28  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/null_overload.exp: New file.
	* gdb.ada/null_overload/foo.adb: New file.

1 files changed, 13 insertions, 2 deletions

diff --git a/gdb/ada-valprint.c b/gdb/ada-valprint.c
index 61c903b..d516a4d 100644
--- a/gdb/ada-valprint.c
+++ b/gdb/ada-valprint.c
@@ -719,6 +719,14 @@ ada_value_print_ptr (struct value *val,
 		     struct ui_file *stream, int recurse,
 		     const struct value_print_options *options)
 {
+  if (!options->format
+      && TYPE_TARGET_TYPE (value_type (val))->code () == TYPE_CODE_INT
+      && TYPE_LENGTH (TYPE_TARGET_TYPE (value_type (val))) == 0)
+    {
+      fputs_filtered ("null", stream);
+      return;
+    }
+
   common_val_print (val, stream, recurse, options, language_def (language_c));
 
   struct type *type = ada_check_typedef (value_type (val));
@@ -1096,8 +1104,11 @@ ada_value_print (struct value *val0, struct ui_file *stream,
   struct type *type = ada_check_typedef (value_type (val));
   struct value_print_options opts;
 
-  /* If it is a pointer, indicate what it points to.  */
-  if (type->code () == TYPE_CODE_PTR)
+  /* If it is a pointer, indicate what it points to; but not for
+     "void *" pointers.  */
+  if (type->code () == TYPE_CODE_PTR
+      && !(TYPE_TARGET_TYPE (type)->code () == TYPE_CODE_INT
+	   && TYPE_LENGTH (TYPE_TARGET_TYPE (type)) == 0))
     {
       /* Hack:  don't print (char *) for char strings.  Their
 	 type is indicated by the quoted string anyway.  */