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

[Jason Merrill <jason@redhat.com>]

On 3/9/20 12:31 PM, Martin Sebor wrote:
> On 2/28/20 1:24 PM, Jason Merrill wrote:
>> 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?
> 
> I'm struggling with how to do this.  Given type (a RECORD_TYPE) and
> type_decl (a TEMPLATE_DECL) representing the use of a template, how
> do I get the corresponding template (or its explicit or partial
> specialization) in the three cases below?
> 
>    1) Instance of the primary:
>       template <class> class A;
>       struct A<int> a;
> 
>    2) Instance of an explicit specialization:
>       template <class> class B;
>       template <> struct B<int>;
>       class B<int> b;
> 
>    3) Instance of a partial specialization:
>       template <class> class C;
>       template <class T> struct C<T*>;
>       class C<int*> c;
> 
> By trial and (lots of) error I figured out that in both (1) and (2),
> but not in (3), TYPE_MAIN_DECL (TYPE_TI_TEMPLATE (type)) returns
> the template's type_decl.
> 
> Is there some function to call to get it in (3), or even better,
> in all three cases?

I think you're looking for most_general_template.

Jason