Use of scoped enum in a class prior to its definition - c++11

Question

I'm trying to understand how the compiler handles this code. The two class methods both reference the enumeration before it is defined, but only one generates a compile error. I'm using GCC 9.2:
class Test {
public:
Test() {}
~Test(){}
//gcc seems to be happy with this...
int Foo()
{
TestState v = TestState::Value1;
if (v == TestState::Value1) {
return 0;
} else if (v == TestState::Value2) {
return 1;
}
}
// ... but it doesn't work when used as a parameter
int Bar(TestState v)
{
if (v == TestState::Value1) {
return 0;
} else if (v == TestState::Value2) {
return 1;
}
}
private:
enum class TestState:bool
{
Value1 = false,
Value2 = true
};
};
In this case, the function Foo compiles fine under gcc 9.2, but Bar does not:
enum.cpp:18:13: error: ‘TestState’ has not been declared
int Bar(TestState v)
^
enum.cpp: In member function ‘int Test::Bar(int)’:
enum.cpp:20:13: error: no match for ‘operator==’ (operand types are ‘int’ and ‘Test::TestState’)
if (v == TestState::Value1) {
^
enum.cpp:22:20: error: no match for ‘operator==’ (operand types are ‘int’ and ‘Test::TestState’)
} else if (v == TestState::Value2) {
Why does gcc seem to accept the usage of the enum prior to definition in one instance but not another? I understand that the enum should be declared before these functions, but why does Foo compile without error?

Answer 1

Maybe you can see this as a lack of declaration of TestState by the time Bar(TestState) is declared. Actually this code can be simplified as below:
struct A {
void f() {} /// fine
void g(B) {} /// not fine
struct B {};
};
int main() {
return 0;
}
This code just can't compile as yours, since by the time g(B) is declared, the compiler needs to look up where the declaration of B is, which cannot be found.
That's when we need Forward Declaration. Add a forward declaration, and you may find compilation successful:
struct A {
void f() {} /// fine
struct B;
void g(B) {} /// fine now
struct B {};
};
int main() {
return 0;
}
It's the same with your code. So add private: enum class TestState : bool; before the public in your code, and it'll compile.