gdb: fix printf of wchar_t early in a gdb session

Given this test program:

  #include <wchar.h>

  const wchar_t wide_str[] = L"wide string";

  int
  main (void)
  {
    return 0;
  }

I observed this GDB behaviour:

  $ gdb -q /tmp/printf-wchar_t
  Reading symbols from /tmp/printf-wchar_t...
  (gdb) start
  Temporary breakpoint 1 at 0x40110a: file /tmp/printf-wchar_t.c, line 8.
  Starting program: /tmp/printf-wchar_t

  Temporary breakpoint 1, main () at /tmp/printf-wchar_t.c:8
  25	  return 0;
  (gdb) printf "%ls\n", wide_str

  (gdb)

Notice that the printf results in a blank line rather than the
expected 'wide string' output.

I tracked the problem down to printf_wide_c_string (in printcmd.c), in
this function we do this:

  struct type *wctype = lookup_typename (current_language,
					 "wchar_t", NULL, 0);
  int wcwidth = wctype->length ();

the problem here is that 'wchar_t' is a typedef.  If we look at the
comment on type::length() we see this:

  /* Note that if thistype is a TYPEDEF type, you have to call check_typedef.
     But check_typedef does set the TYPE_LENGTH of the TYPEDEF type,
     so you only have to call check_typedef once.  Since value::allocate
     calls check_typedef, X->type ()->length () is safe.  */

What this means is that after calling lookup_typename we should call
check_typedef in order to ensure that the length of the typedef has
been setup correctly.  We are not doing this in printf_wide_c_string,
and so wcwidth is incorrectly calculated as 0.  This is what leads GDB
to print an empty string.

We can see in c_string_operation::evaluate (in c-lang.c) an example of
calling check_typedef specifically to fix this exact issue.

Initially I did fix this problem by adding a check_typedef call into
printf_wide_c_string, but then I figured why not move the
check_typedef call up into lookup_typename itself, that feels like it
should be harmless when looking up a non-typedef type, but will avoid
bugs like this when looking up a typedef.  So that's what I did.

I can then remove the extra check_typedef call from c-lang.c, I don't
see any other places where we had extra check_typedef calls.  This
doesn't mean we definitely had bugs -- so long as we never checked the
length, or, if we knew that check_typedef had already been called,
then we would be fine.

I don't see any test regressions after this change, and my new test
case is now passing.

Reviewed-By: Tom Tromey <tom@tromey.com>

gdb/gdbtypes.c

@@ -1648,9 +1648,7 @@ type_name_or_error (struct type *type)
 	 objfile ? objfile_name (objfile) : "<arch>");
 }
 
-/* Lookup a typedef or primitive type named NAME, visible in lexical
-   block BLOCK.  If NOERR is nonzero, return zero if NAME is not
-   suitably defined.  */
+/* See gdbtypes.h.  */
 
 struct type *
 lookup_typename (const struct language_defn *language,
@@ -1662,7 +1660,12 @@ lookup_typename (const struct language_defn *language,
   sym = lookup_symbol_in_language (name, block, VAR_DOMAIN,
 				   language->la_language, NULL).symbol;
   if (sym != NULL && sym->aclass () == LOC_TYPEDEF)
-    return sym->type ();
+    {
+      struct type *type = sym->type ();
+      /* Ensure the length of TYPE is valid.  */
+      check_typedef (type);
+      return type;
+    }
 
   if (noerr)
     return NULL;