[basic.link]9 (emphasis is mine):

When a block scope declaration of an entity with linkage is not found to refer to some other declaration, then that entity is a member of the innermost enclosing namespace. However such a declaration does not introduce the member name in its namespace scope.

[basic.scope.declarative]/4 (emphasis is mine):

[ Note: These restrictions apply to the declarative region into which a name is introduced, which is not necessarily the same as the region in which the declaration occurs. In particular, elaborated-type-specifiers and friend declarations may introduce a (possibly not visible) name into an enclosing namespace; these restrictions apply to that region. Local extern declarations ([basic.link]) may introduce a name into the declarative region where the declaration appears and also introduce a (possibly not visible) name into an enclosing namespace; these restrictions apply to both regions. — end note ]