Proposed resolution (approved by CWG 2023-02-09): Add to 9.13.1 [dcl.attr.grammar] paragraph 6 as follows:
[Note 4: A program is ill-formed if it contains an attribute
specified in 9.13 [dcl.attr] that violates the rules
specifying to which entity or statement the attribute can apply or the
syntax rules for the attribute's attribute-argument-clause, if
any. —end note]
[Note: The attributes specified in
9.13 [dcl.attr] have optional semantics: given a well-formed
program, removing all instances of any one of those attributes
results in a program whose set of possible executions
(4.1.2 [intro.abstract]) for a given input is a subset of those of
the original program for the same input, absent implementation-defined
guarantees with respect to that attribute. -- end note ]