Template argument deduction for class templates in C++17: am I doing it wrong?

Question

According to https://gcc.gnu.org/projects/cxx-status.html, version 7 of g++, used with flag -std=c++1z, supports template argument deduction for class templates

Answer 1

Class template argument deduction works for declaring instances of class types:

Derived d(42);

Or new-expressions:

auto p = new Derived(42);

Or function-style casts:

foo(Derived(42));

It does not work for declaring pointers.

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You'll have to simply provide the template arguments as you've always had to. Or, I guess:

template <class T> Base<T>* downcast(Base<T>* p) { return p; }
auto pt_base = downcast(new Derived(42));