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

[John McCall via cfe-dev]

> 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 <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 <mailto: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 <mailto: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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