Class template argument deduction and default template parameters

Question

The following stripped down code doesn\'t work with the latest clang++5 but is accepted by g++7:

template
struct wrapper;

te         


        

Answer 1

This is not a quote of the Standard per se¹, but I feel confident enough to consider it an answer.

According to cppreference, on Default template arguments:

Default template arguments that appear in the declarations and the definition are merged similarly to default function arguments:

template<typename T1, typename T2 = int> class A;
template<typename T1 = int, typename T2> class A;
// the above is the same as the following:
template<typename T1 = int, typename T2 = int> class A;

But the same parameter cannot be given default arguments twice in the same scope

template<typename T = int> class X;
template<typename T = int> class X {}; // error

This implies an implicit rule: A template type argument can be given a default type in the template declaration or template definition interchangeably.

The behavior clang++5 exhibits is definitly a bug.

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¹⁾ Provided by user Oliv:

[temp.param]/10

The set of default template-arguments available for use is obtained by merging the default arguments from all prior declarations of the template in the same way default function arguments are ([dcl.fct.default]). [ Example:

template<class T1, class T2 = int> class A;
template<class T1 = int, class T2> class A;

is equivalent to

template<class T1 = int, class T2 = int> class A;

 — end example ]