Is it required to explicitly list default parameters when using template template parameter?
I\'d like to ask whether the following code sample should compile:
#include
#include
#include
using namespa
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A (seemingly related) quote from the standard
14.1 Template parameters14 . A template-parameter of a template template-parameter is permitted to have a default template-argument. When such default arguments are specified, they apply to the template template-parameter in the scope of the template template-parameter.
[ Example:
templatestruct B {}; template class T> struct A { inline void f(); inline void g(); }; template class T> void A ::f() { // (*) T<> t; // error - TT has no default template argument } template class T> void A ::g() { T<> t; // OK - T } — end example ]
This is the only verse posing limitations to the use of default template parameters of template template parameters (verse 9,11,12 pose limitations on the definition/specification)
As stressed in the comments, OP's case does not involve a default parameter in
convert_container(so the above does not apply explicitly). IMHO there are two ways of interpreting the situation :using type = Cis a type alias for a class template; that class "loses" the right to use default template parameters, since it's passed as a template template parameter and all default arguments (of that TT parameter) lie outside the scope of the "typedefing". Then VS is correct.By tracking the instantiation process : Say a correct compiler instantiates the struct as so (it's just a type substitution - no actual representation of the actual instantiation process is implied)
struct convert_container { using type = vector; };
then OP's case seems fairly legit (and gcc/clang are correct)
FWIW
This compiles in VS2013
template class C> using tt = C; int main() { std::cout << typeid(tt So the arguments of default template parameters being non legal to pass to template template parameters seems more and more shaky.
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