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.h

@@ -2586,8 +2586,18 @@ extern void check_stub_method_group (struct type *, int);
 
 extern char *gdb_mangle_name (struct type *, int, int);
 
-extern struct type *lookup_typename (const struct language_defn *,
-				     const char *, const struct block *, int);
+/* 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.
+
+   If this function finds a suitable type then check_typedef is called on
+   the type, this ensures that if the type being returned is a typedef
+   then the length of the type will be correct.  The original typedef will
+   still be returned, not the result of calling check_typedef.  */
+
+extern struct type *lookup_typename (const struct language_defn *language,
+				     const char *name,
+				     const struct block *block, int noerr);
 
 extern struct type *lookup_template_type (const char *, struct type *,
 					  const struct block *);