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

[Dimitry Andric via cfe-dev]

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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