9 Declarations [dcl.dcl]

9.12 Attributes [dcl.attr]

9.12.1 Attribute syntax and semantics [dcl.attr.grammar]

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If an attribute-specifier contains an attribute-using-prefix, the attribute-list following that attribute-using-prefix shall not contain an attribute-scoped-token and every attribute-token in that attribute-list is treated as if its identifier were prefixed with N​::​, where N is the attribute-namespace specified in the attribute-using-prefix.
[Note 1: 
This rule imposes no constraints on how an attribute-using-prefix affects the tokens in an attribute-argument-clause.
— end note]
[Example 1: [[using CC: opt(1), debug]] // same as [[CC​::​opt(1), CC​::​debug]] void f() {} [[using CC: opt(1)]] [[CC::debug]] // same as [[CC​::​opt(1)]] [[CC​::​debug]] void g() {} [[using CC: CC::opt(1)]] // error: cannot combine using and scoped attribute token void h() {} — end example]
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[Note 2: 
For each individual attribute, the form of the balanced-token-seq will be specified.
— end note]
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In an attribute-list, an ellipsis may appear only if that attribute's specification permits it.
An attribute followed by an ellipsis is a pack expansion.
An attribute-specifier that contains no attributes has no effect.
The order in which the attribute-tokens appear in an attribute-list is not significant.
If a keyword or an alternative token that satisfies the syntactic requirements of an identifier ([lex.name]) is contained in an attribute-token, it is considered an identifier.
No name lookup is performed on any of the identifiers contained in an attribute-token.
The attribute-token determines additional requirements on the attribute-argument-clause (if any).
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Each attribute-specifier-seq is said to appertain to some entity or statement, identified by the syntactic context where it appears ([stmt.stmt], [dcl.dcl], [dcl.decl]).
If an attribute-specifier-seq that appertains to some entity or statement contains an attribute or alignment-specifier that is not allowed to apply to that entity or statement, the program is ill-formed.
If an attribute-specifier-seq appertains to a friend declaration ([class.friend]), that declaration shall be a definition.
[Note 3: 
An attribute-specifier-seq cannot appeartain to an explicit instantiation ([temp.explicit])