I am trying to use a std::bitset with an enum but I am getting a compilation error saying
template argument 1 is invalid
Funny thing is that when I use any of the enumerated value without the enumeration scope it works fine.
Do you know why?
Below the code
enum MyTypes { Alpha = 1, Beta = 2, Gamma = 3 }; std::bitset<MyTypes::Alpha> bitset_wrong; // It doesn't compile. std::bitset<Alpha > bitset_good; // It works. 
It seems that you have an old compiler that does not support specifying qualified names with unscoped enumerators.
Update your compiler.:)
The code you showed is a valid code according to the C++ 2011 Standard.
Here is a quote from the C++ Standard with an example (7.2 Enumeration declarations)
11 Each enum-name and each unscoped enumerator is declared in the scope that immediately contains the enum-specifier. Each scoped enumerator is declared in the scope of the enumeration. These names obey the scope rules defined for all names in (3.3) and (3.4).
[ Example:
enum direction { left=’l’, right=’r’ }; void g() { direction d; // OK d = left; // OK d = direction::right; // OK } 
-std=c++11in order to use C++11 features.enum struct MyTypes{...}work as a scoped enumerator? I'm not sure if unscoped enumerators support qualified names by default, see en.cppreference.com/w/cpp/language/enum : Each enumerator becomes a named constant of the enumeration's type (that is, name), visible in the enclosing scope...MyTypesis neither, so it would be an error to try and use it as a qualifier.