11 Classes [class]

11.4 Class members [class.mem]

11.4.11 Nested type names [class.nested.type]

1
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Type names obey exactly the same scope rules as other names.
In particular, type names defined within a class definition cannot be used outside their class without qualification.
[Example: 
struct X {
  typedef int I;
  class Y { /* ... */ };
  I a;
};

I b;                            // error
Y c;                            // error
X::Y d;                         // OK
X::I e;                         // OK
— end example]