E2436 Default type for template template argument 'arg' does not name a primary template class (C++)
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If a template template parameter is to have a default type, that type must either be a generic template class name, or another template template parameter.
template<class T> class foo; template<template<class T> class U = foo> class bar;// OK: "foo" is a qualifying primary template template<template<class T> class U = int> class baz;// error: "int" is not a template class