[PATCH] Fix return type deduction for member templates

[Richard Smith]

On Thu, Jun 13, 2013 at 5:03 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?

Yes, that's fine.