Trevor Hickey Trevor Hickey - 3 months ago 13
C++ Question

g++-6 errors on shadowed template parameters while g++-5 does not

Take the following example:

#include <vector>

template <typename T, template <class T> class Container>
std::vector<T> To_Vector(Container<T> const& c){
return std::vector<T>( c.begin(), c.end() );
}

int main(){}


With
g++-5
, it compiles without errors:

g++-5 -o main main.cpp


With
g++-6
is fails to compile:

g++-6 -o main main.cpp
main.cpp:4:33: error: declaration of template parameter ‘T’ shadows template parameter
template <typename T, template <class T> class Container>
^~~~~
main.cpp:4:11: note: template parameter ‘T’ declared here
template <typename T, template <class T> class Container>


Is the compiler wrong? Is my code wrong?

Why does
g++-5
compile this code while
g++-6
does not?




g++-5 --version
g++-5 (Ubuntu 5.4.1-2ubuntu1~14.04) 5.4.1 20160904
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.





g++-6 --version
g++-6 (Ubuntu 6.2.0-3ubuntu11~14.04) 6.2.0 20160901
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Answer

The behaviour of g++6 is correct because according to the standard:

A template-parameter shall not be redeclared within its scope (including nested scopes). A template-parameter shall not have the same name as the template name.

The scope of T begins right after its declaration, so it extends into the following template template parameter, which redeclares T as a template parameter, therefore this rule is violated.

I imagine the change between g++5 and g++6 was due to fixing some bug report around a similar issue.