Inherit from class template nested in template argument

Question

I have a trait class that specifies base classes and derived classes for a class hierarchy. I'm trying to inherit from a class template nested inside the trait class.

Here is a tiny example:

template<typename GraphTypes, bool IsNode>
class Graph : public GraphTypes::BaseGraph<IsNode>
{};

class MyGraphTypes {
public:
    template<bool IsNode>
    class BaseGraph {};
};

Graph<MyGraphTypes, true> myGraph;

It compiles on GCC, but not on Clang or MSVC:

• MSVC: error C2143: syntax error: missing ',' before '<'
• Clang: error: missing 'template' keyword prior to dependent template name 'GraphTypes::BaseGraph'

I really need it in MSVC. Is it a bug or limitation, or is there something I can do to make MSVC understand the code?

Here you can try it out: https://godbolt.org/z/8rTzGzxPG

Answer 1

Clang provides the better error message here than MSVC.

BaseGraph is a dependent type because it depends on another template parameter. For these you need to add an additional typename like this:

template<typename GraphTypes, bool IsNode>
class Graph : public GraphTypes::template BaseGraph<IsNode>
{};

This compiles fine on MSVC as well.