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

[Martin Sebor]

On 3/9/20 1:40 PM, Jason Merrill wrote:
> 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.

I don't think that's quite what I'm looking for.  At least it doesn't
return the template or its specialization in all three cases above.
In (2) and (3) it won't distinguish between specializations of B or
C on different types.  In (2), the function returns the same result
for both:

   template <> struct B<int>;
   template <> struct B<char>;

In (3), it similarly returns the same result for both of

   template <class T> struct C<T*>;
   template <class T> struct C<const T>;

even though they are declarations of distinct types.

What I missing?

Martin