[Bug c++/88221] New: Only restricted set of balanced token sequences allowed for attributes

[o_kniemeyer at maxon dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88221

            Bug ID: 88221
           Summary: Only restricted set of balanced token sequences
                    allowed for attributes
           Product: gcc
           Version: 8.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: o_kniemeyer at maxon dot net
  Target Milestone: ---

Attribute arguments may be arbitrary token sequences as long as they are
balanced regarding () [] {}. From C++17 10.6.1:

attribute-argument-clause :
    ( balanced-token-seq opt )

balanced-token-seq :
    balanced-token
    balanced-token-seq balanced-token

balanced-token :
    ( balanced-token-seqopt )
    [ balanced-token-seqopt ]
    { balanced-token-seqopt }
    any token other than a parenthesis, a bracket, or a brace

However GCC only allows a quite restricted set. For example the following
arguments are rejected:

[[myattribute(a)]] void Func();
[[myattribute(void)]] void Func();
[[myattribute(1 2)]] void Func();
[[myattribute(1:2)]] void Func();
[[myattribute([1])]] void Func();