namespace test
{
    enum ETest { A, B }
    
    class CTest {
        CTest(const ETest v = ETest::A) {}
    }
}

void main()
{
  test::CTest a;
}

Prepare the above code and execute it.

> asrun.exe script2.as
script2.as (15, 1) : INFO : Compiling void main()
default arg (1, 1) : ERR  : Unknown scope 'ETest'
script2.as (17, 15) : ERR  : Failed while compiling default arg for parameter 0 in function 'test::CTest@ CTest(const test::ETest = ETest :: A)'

Abnormally ends after outputting the above.

Describing the namespace as below will compile successfully.

CTest(const ETest v = test::ETest::A) {}