[PATCH] Fix return type deduction for member templates

[Richard Smith]

On Thu, Jun 13, 2013 at 5:11 PM, Faisalv <faisalv at gmail.com> wrote:
> 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)

It should be fine to call DiagnoseUseOfDecl on both if they're
different; they're both "used" in some sense.

> 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?