ラベル名と識別子名

プログラミング言語C/C++のラベル(label)名は、他の識別子名（関数名や変数名など）と独立した名前空間*1を持つ。

void f()
{
  /* foo statement */;

std:  // OK
  /* bar statement */;
}

C++03 6.1/p1より該当箇所を引用。

(snip) Labels have their own name space and do not interfere with other identifiers.

C99 6.2.3/p1より該当箇所を引用。

If more than one declaration of a particular identifier is visible at any point in a translation unit, the syntactic context disambiguates uses that refer to different entities. Thus, there are separate name spaces for various categories of identifiers, as follows:

• label names (disambiguated by the syntax of the label declaration and use);

関連URL

• What would 'std:;' do in c++? - Stack Overflow

*1:C++の namespace キーワードが提供する機能とは別物。