Is there an equivalent of "typename" for template types?

Question

We have a template member function of the form:

template <template <class> class TT>
TT<some_type> foo() const;

Now, in an invocation context where TT is explicitly specified from a dependent name:

template <class T_other>
void bar()
{
    instance.foo<T_other::template_type>();
}

Visual Studio is able to compile the invocation. GCC complains:

error: dependent-name ‘SomeConcreteType::template_type’ is parsed as a non-type, but instantiation yields a type.

And advises to say typename SomeConcreteType::template_type. Which does not work, since SomeConcreteType::template_type is a template type, not a plain type.

Is there a keyword that can be used here?
Another way to make this accepted by the big 3 compilers?

EDIT: Now with a Godbolt minimal example https://godbolt.org/z/hvbb6jv9W

Can we see the definition of something that would qualify as a `T_other`? — Nicol Bolas, Jul 22 '21 at 15:20
@NathanOliver As pointed out, template_type is not a type, but a template (type), so it is not accepted. — Ad N, Jul 22 '21 at 15:22
Can we get a [mre] with the code using `typename SomeConcreteType::template_type`? — NathanOliver, Jul 22 '21 at 15:22
I will work on extracting the minimal example, I hoped it was simpler. — Ad N, Jul 22 '21 at 15:23
@NathanOliver minimal example https://godbolt.org/z/hvbb6jv9W — Ad N, Jul 22 '21 at 15:34
@PatrickRoberts Indeed, thank you for catching this mistake! — Ad N, Jul 22 '21 at 15:41
I put the duplicate back. This is the exact same problem. You have to put `template` in front of a dependent name that must be a template. — Guillaume Racicot, Jul 22 '21 at 16:01
@GuillaumeRacicot Fair enough, I needed this discussion to better grasp that it was the underlying issue. — Ad N, Jul 23 '21 at 07:47

score 2 · Accepted Answer · answered Jul 22 '21 at 15:30

2

Use template instead of typename. And, note that while typename would go before the ::, template goes after.

template<class T_other>
void bar() {
    instance.foo<T_other::template template_type>();
}

Complete example.

answered Jul 22 '21 at 15:30

HTNW

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And you provided the solution before the minimal example arrived, congratulations! – Ad N Jul 22 '21 at 15:37

Is there an equivalent of "typename" for template types?

1 Answers1