An elaborated-type-specifier ([dcl.type.elab]) may be used to refer to a previously declared class-name or enum-name even though the name has been hidden by a non-type declaration ([basic.scope.hiding]).
If the elaborated-type-specifier has no nested-name-specifier, and unless the elaborated-type-specifier appears in a declaration with the following form:
class-key attribute-specifier-seqopt identifier ;
the identifier is looked up according to [basic.lookup.unqual] but ignoring any non-type names that have been declared. If the elaborated-type-specifier is introduced by the enum keyword and this lookup does not find a previously declared type-name, the elaborated-type-specifier is ill-formed. If the elaborated-type-specifier is introduced by the class-key and this lookup does not find a previously declared type-name, or if the elaborated-type-specifier appears in a declaration with the form:
class-key attribute-specifier-seqopt identifier ;
the elaborated-type-specifier is a declaration that introduces the class-name as described in [basic.scope.pdecl].
If the elaborated-type-specifier has a nested-name-specifier, qualified name lookup is performed, as described in [basic.lookup.qual], but ignoring any non-type names that have been declared. If the name lookup does not find a previously declared type-name, the elaborated-type-specifier is ill-formed. [ Example:
struct Node {
struct Node* Next; // OK: Refers to Node at global scope
struct Data* Data; // OK: Declares type Data
// at global scope and member Data
};
struct Data {
struct Node* Node; // OK: Refers to Node at global scope
friend struct ::Glob; // error: Glob is not declared
// cannot introduce a qualified type ([dcl.type.elab])
friend struct Glob; // OK: Refers to (as yet) undeclared Glob
// at global scope.
/* ... */
};
struct Base {
struct Data; // OK: Declares nested Data
struct ::Data* thatData; // OK: Refers to ::Data
struct Base::Data* thisData; // OK: Refers to nested Data
friend class ::Data; // OK: global Data is a friend
friend class Data; // OK: nested Data is a friend
struct Data { /* ... */ }; // Defines nested Data
};
struct Data; // OK: Redeclares Data at global scope
struct ::Data; // error: cannot introduce a qualified type ([dcl.type.elab])
struct Base::Data; // error: cannot introduce a qualified type ([dcl.type.elab])
struct Base::Datum; // error: Datum undefined
struct Base::Data* pBase; // OK: refers to nested Data