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

[Jason Merrill <jason@redhat.com>]

On 2/28/20 12:45 PM, Martin Sebor wrote:
> 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).
>>
>>> +         a new entry for it and return unless it's a declaration
>>> +         involving a template that may need to be diagnosed by
>>> +         -Wredundant-tags.  */
>>>        *rdl = class_decl_loc_t (class_key, false, def_p);
>>> -      return;
>>> +      if (TREE_CODE (decl) != TEMPLATE_DECL)
>>> +        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

When we get here for an instance of a template, it doesn't make sense to 
treat it as a new type.

If decl is a template and type_decl is an instance of that template, do 
we want to (before the lookup) change type_decl to the template or the 
corresponding generic TYPE_DECL, which should already be in the table?

Jason