[PATCH] avoid -Wredundant-tags on a first declaration in use (PR 93824)

[Martin Sebor]

On 2/28/20 9:58 AM, Jason Merrill wrote:
> On 2/24/20 6:58 PM, Martin Sebor wrote:
>> -Wredundant-tags doesn't consider type declarations that are also
>> the first uses of the type, such as in 'void f (struct S);' and
>> issues false positives for those.  According to the reported that's
>> making it harder to use the warning to clean up LibreOffice.
>>
>> The attached patch extends -Wredundant-tags to avoid these false
>> positives by relying on the same class_decl_loc_t::class2loc mapping
>> as -Wmismatched-tags.  The patch also somewhat improves the detection
>> of both issues in template declarations (though more work is still
>> needed there).
> 
>> +Â Â Â Â Â Â Â Â  a new entry for it and return unless it's a declaration
>> +Â Â Â Â Â Â Â Â  involving a template that may need to be diagnosed by
>> +         -Wredundant-tags.  */
>> Â Â Â Â Â Â  *rdl = class_decl_loc_t (class_key, false, def_p);
>> -Â Â Â Â Â  return;
>> +Â Â Â Â Â  if (TREE_CODE (decl) != TEMPLATE_DECL)
>> +Â Â Â Â Â Â Â  return;
> 
> How can the first appearance of a class template be redundant?

I'm not sure I correctly understand the question.  The comment says
"involving a template" (i.e., not one of the first declaration of
a template).  The test case that corresponds to this test is:

   template <class> struct S7 { };
   struct S7<void> s7v;  // { dg-warning "\\\[-Wredundant-tags" }

where DECL is the TEPLATE_DECL of S7<void>.

As I mentioned, more work is still needed to handle templates right
because some redundant tags are still not diagnosed.  For example:

   template <class> struct S7 { };
   template <class T>
   using U = struct S7<T>;   // missing warning

Martin