Why does this code compile with MSVC, but not in GCC or Clang?-CodePudding

And how to fix the code?

Here is the code: https://godbolt.org/z/vcP6WKvG5

#include <memory>
#include <utility>

enum class Format {
    Number,
    Text,    
};

template <template <Format> typename Visitor, typename... Args>
void switchByFormat(Format format, Args&&... args) {
    switch(format) {
        case Format::Number: 
            Visitor<Format::Number>::visit(std::forward<Args>(args)...);
            break;
        case Format::Text: 
            Visitor<Format::Text>::visit(std::forward<Args>(args)...);
            break;
    }
}

struct AstNode {};

template <Format format>
struct ANode: public AstNode {};

using AstNodePtr = std::shared_ptr<AstNode>;

template <template <Format> typename AstNodeT, 
          typename... Args>
struct AstNodeFactory {
    template <Format format>
    struct Visitor {
        static void visit(AstNodePtr& result, Args&&... args)
        {
            result = std::make_shared<AstNodeT<format>>(std::forward<Args>(args)...);
        }
    };
};

template <template <Format> typename AstNodeT,              
          typename... Args>
AstNodePtr makeAstNode(Format format, Args&&... args)
{
    AstNodePtr result;
    switchByFormat<typename AstNodeFactory<AstNodeT, Args...>::Visitor>(format,
                                                                result,
                                                                std::forward<Args>(args)...);
    return result;
}

int main() {
    auto textAnode = makeAstNode<ANode>(Format::Text);
}

Clang link: https://godbolt.org/z/9fxWE9j71

The error in Clang is:

source>:45:64: error: typename specifier refers to class template member in 'AstNodeFactory<ANode>'; argument deduction not allowed here
    switchByFormat<typename AstNodeFactory<AstNodeT, Args...>::Visitor>(format,

GCC link: https://godbolt.org/z/4EvrzGWYE

GCC error:

In instantiation of 'AstNodePtr makeAstNode(Format, Args&& ...) [with AstNodeT = ANode; Args = {}; AstNodePtr = std::shared_ptr<AstNode>]':
<source>:52:53:   required from here
<source>:45:5: error: 'typename AstNodeFactory<ANode>::Visitor' names 'template<enum class Format format> struct AstNodeFactory<ANode>::Visitor', which is not a type
   45 |     switchByFormat<typename AstNodeFactory<AstNodeT, Args...>::Visitor>(format,
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

CodePudding user response：

Since Visitor is a template class rather than a type, you need to specify the template keyword

switchByFormat<AstNodeFactory<AstNodeT, Args...>::template Visitor>(
                                                //^^^^^^^^
  format, result, std::forward<Args>(args)...);

Demo.