The braces indicate a constructor call, but there is none. There is a non-member helper template though, but it has no body. So, what's happening here?
It works also without deduction guide and the 'using' fix is for g++. 'operator()' cannot be ambiguous in visitors but gcc is very careful with overloading derived functions. In clang we can easily have the same results with minimal code (checked output assembler with Compiler Explorer):
I'm confused with this part:
The braces indicate a constructor call, but there is none. There is a non-member helper template though, but it has no body. So, what's happening here?
It's not a non-member helper template, it's a user defined deduction guide. See this article:
arne-mertz.de/2017/06/class-templa...
It works also without deduction guide and the 'using' fix is for g++. 'operator()' cannot be ambiguous in visitors but gcc is very careful with overloading derived functions. In clang we can easily have the same results with minimal code (checked output assembler with Compiler Explorer):
There is needed aggregate Initialisation + variadic templates.