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c++templatestemplate-templates

Is default template template parameter value deduced context?

I was actually surprised that both gcc and clang accept this code:

#include <iostream>
#include <vector>
#include <type_traits>

template <class T, template <class, class = T> class TT, class Y>
T foo(TT<Y>) {
}

int main() {
    static_assert(std::is_same<decltype(foo(std::vector<int>{})), std::allocator<int>>::value);
}

Are gcc and clang right that the values of the default template template parameters are deduced context or is it compilers extension?

Solution

  • When you write

    template <class T, template <class, class = T> class TT, class Y>
T foo(TT<Y>);

    it is equivalent to

    template <class T, template <class, class = T> class TT, class Y>
T foo(TT<Y, T>);

    So T can be deduced.