CWG3038 Ignorability of attributes, again #2407

The following note added to 9.13.1 [dcl.attr.grammar] by issue 2695 embodies the design intent that attributes can be ignored by implementations:

[Note 5: 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]

However, the note is incorrect for attributes that contain a potentially-evaluated expression such as [[assume]] (9.13.3 [dcl.attr.assume]).

See CWG3038 for details.

@timuraudio