[cfe-dev] Potential bug in class template argument deduction

[Hamza Sood via cfe-dev]

Hi John,

I’ve filed a bug here: https://bugs.llvm.org/show_bug.cgi?id=34195

> On 15 Aug 2017, at 15:50, John McCall <rjmccall at apple.com> wrote:
> 
>> On Aug 15, 2017, at 7:00 AM, Hamza Sood via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>> The GCC C++ status page (https://gcc.gnu.org/projects/cxx-status.html) shows that class template argument deduction wasn’t implemented until GCC-7.
>> 
>> However Clang’s status page also reports it as implemented (in SVN builds), but Clang built from SVN trunk rejects that code.
> 
> The template-argument inference logic in the proposed standard has undergone repeated late revisions; I expect that both GCC and Clang implement some intermediate version that isn't quite fully what's specified in the current draft.  Of course that's something we should fix.  Please file a bug on bugs.llvm.org and we'll take a look.
> 
> John.
> 
>> 
>>> On 15 Aug 2017, at 11:53, Dimitry Andric wrote:
>>> 
>>> Hm, it appears to depend on the gcc version:
>>> 
>>> $ g++6 -std=c++17 test.cpp
>>> test.cpp: In function 'int main()':
>>> test.cpp:8:9: error: missing template arguments before 'b'
>>>    Base b(d);
>>>         ^
>>> 
>>> However, gcc 7.1.1 does accept it.  So maybe this is something very new?
>>> 
>>> -Dimitry
>>> 
>>>> On 15 Aug 2017, at 12:46, Hamza Sood <hamza_sood at me.com> wrote:
>>>> 
>>>> Sorry, I should’ve been more clear. I’m talking about C++17 class template argument deduction.
>>>> 
>>>> If you pass “-std=c++17” to GCC, then it compiles the code fine. However Clang still rejects it.
>>>> 
>>>> (Sending again because I forgot to reply all)
>>>> 
>>>>>> On 15 Aug 2017, at 11:40, Dimitry Andric wrote:
>>>>>> 
>>>>>> On 15 Aug 2017, at 12:04, Hamza Sood via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>>>>>> 
>>>>>> template<typename T>
>>>>>> class Base { };
>>>>>> 
>>>>>> class Derived: public Base<int> { };
>>>>>> 
>>>>>> int main() {
>>>>>> Derived d;
>>>>>> Base b(d);
>>>>>> return 0;
>>>>>> }
>>>>>> 
>>>>>> 
>>>>>> Clang rejects that code, saying it can’t deduce the template parameters for Base.
>>>>>> However I think that code is valid; it’s accepted by GCC.
>>>>> 
>>>>> Not here, with gcc 6.4.0:
>>>>> 
>>>>> $ g++ -c test.cpp
>>>>> test.cpp: In function 'int main()':
>>>>> test.cpp:8:9: error: missing template arguments before 'b'
>>>>> Base b(d);
>>>>>      ^
>>>>> 
>>>>> The declaration should use Base<int> instead.
>>>>> 
>>>>> 
>>>>>> Is this a Clang bug?
>>>>> 
>>>>> No, though clang's error message is substantially more confusing, in my opinion:
>>>>> 
>>>>> $ clang++ -c test.cpp
>>>>> test.cpp:8:4: error: unknown type name 'Base'; did you mean 'Derived::Base'?
>>>>> Base b(d);
>>>>> ^~~~
>>>>> Derived::Base
>>>>> test.cpp:2:7: note: 'Derived::Base' declared here
>>>>> class Base { };
>>>>>   ^
>>>>> 
>>>>> It should give a similar suggestion as gcc does.
>>>>> 
>>>>> -Dimitry
>>>>> 
>>> 
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