9 Declarations [dcl.dcl]

9.2 Specifiers [dcl.spec]

9.2.8 Type specifiers [dcl.type]

9.2.8.3 Elaborated type specifiers [dcl.type.elab]

elaborated-type-specifier:
	class-key attribute-specifier-seq nested-name-specifier identifier
	class-key simple-template-id
	class-key nested-name-specifier template simple-template-id
	elaborated-enum-specifier

elaborated-enum-specifier:
	enum nested-name-specifier identifier
1
#
An attribute-specifier-seq shall not appear in an elaborated-type-specifier unless the latter is the sole constituent of a declaration.
If an elaborated-type-specifier is the sole constituent of a declaration, the declaration is ill-formed unless it is an explicit specialization ([temp.expl.spec]), an explicit instantiation ([temp.explicit]) or it has one of the following forms:
class-key attribute-specifier-seq identifier ;
friend class-key :: identifier ;
friend class-key :: simple-template-id ;
friend class-key nested-name-specifier identifier ;
friend class-key nested-name-specifier template simple-template-id ;
In the first case, the attribute-specifier-seq, if any, appertains to the class being declared; the attributes in the attribute-specifier-seq are thereafter considered attributes of the class whenever it is named.
2
#
[ Note
:
[basic.lookup.elab] describes how name lookup proceeds for the identifier in an elaborated-type-specifier.
— end note
 ]
If the identifier or simple-template-id resolves to a class-name or enum-name, the elaborated-type-specifier introduces it into the declaration the same way a simple-type-specifier introduces its type-name.
[ Note
:
This implies that, within a class template with a template type-parameter T, the declaration 
friend class T;
is ill-formed.
However, the similar declaration friend T; is allowed ([class.friend]).
— end note
 ]
3
#
The class-key or enum keyword present in the elaborated-type-specifier shall agree in kind with the declaration to which the name in the elaborated-type-specifier refers.
This rule also applies to the form of elaborated-type-specifier that declares a class-name or friend class since it can be construed as referring to the definition of the class.
Thus, in any elaborated-type-specifier, the enum keyword shall be used to refer to an enumeration ([dcl.enum]), the union class-key shall be used to refer to a union ([class.union]), and either the class or struct class-key shall be used to refer to a non-union class ([class.pre]).
[ Example
: 
enum class E { a, b };
enum E x = E::a;                // OK
struct S { } s;
class S* p = &s;                // OK
— end example
 ]