[PATCH] Fix return type deduction for member templates

[Faisalv]

Also can i just check if the founddecl and the decl being used are different - and if so, call diagnoseusedecl on both - or does one have to be a specialization of the other? (do u have an example in mind where it is not a specilaization)

Thanks!

Sent from my iPhone

On Jun 13, 2013, at 8:03 PM, Faisalv <faisalv at gmail.com> wrote:

> 
> 
> Sent from my iPhone
> 
> On Jun 13, 2013, at 7:36 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> 
>> On Thu, Jun 13, 2013 at 3:40 PM, Faisalv <faisalv at gmail.com> wrote:
>>> 
>>> 
>>> On Jun 13, 2013, at 4:30 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>>> 
>>>> Hi Faisal,
>>>> 
>>>> On Thu, Jun 13, 2013 at 11:30 AM, Faisal Vali <faisalv at gmail.com> wrote:
>>>>> While implementing return type deduction for generic lambdas, I stumbled upon
>>>>> a bug which led to clang mishandling the following code:
>>>>> 
>>>>> struct Lambda {
>>>>> template<class T> auto operator()(T t) {
>>>>>  return 5;
>>>>> }
>>>>> template<class T> auto foo(T t) { return t; }
>>>>> };
>>>>> void test() {
>>>>>  Lambda L;
>>>>>  int i = L(3);
>>>>> }
>>>>> 
>>>>> 
>>>>> The issue (as i understand it) was that in some contexts only the
>>>>> function template (and not the specialization) would get passed to
>>>>> DiagnoseUseOfDecl (which deduces the return type of the function).  So
>>>>> the specialization would flow through to Codegen with auto as its
>>>>> return type.
>>>>> 
>>>>> So, with that in mind, this patch implements a trivial fix.
>>>>> 
>>>>> What do you think Richard? Is there a better way to address this bug?
>>>> 
>>>> I was aware of this issue, but got snowed under with other things.
>>>> This bug is not limited to 'auto' type deduction; the other code in
>>>> DiagnoseUseOfDecl suffers the same way. For instance:
>>>> 
>>>> struct Lambda {
>>>> template<class T> static __attribute__((unused)) int foo(T) {
>>>>  return 5;
>>>> }
>>>> };
>>>> int bar() {
>>>>  Lambda L;
>>>>  return L.foo(3);
>>>> }
>>>> 
>>>> ... does not warn that the function is marked 'unused' but is used,
>>>> but if you call it as Lambda::foo(3), it does warn.
>>>> 
>>>> I think the right fix here is to pass both the declaration found by
>>>> name lookup and the declaration that is actually used into
>>>> DiagnoseUseOfDecl.
>>>> 
>>> 
>>> What if i just call diagnoseuseofdecl again on the used specialization instead of adding a parameter to it?
>> 
>> If this is the only call which has the problem, then that would be OK.
>> I suspect there are other cases, though, and there are probably cases
>> which have the opposite bug (passing only the used decl and not the
>> found decl). As a quick fix, making two calls would work, but it's
>> probably not what we want in the longer term.
> 
> 
> If its ok w u, for now id rather do the quick fix, place the appropriate fixmes - and then try and return to this once generic lambdas are starting to be reasonably usable (plus i still have to figure out that pesky non-odr use, rvalue emission issue - which also i'm going to deprioritize a lil in favor of usable generic lambdas for now). Thoughts?