[Bug c++/66478] New: [constexpr] accepts-invalid with read of non-constant variable as discarded value

[richard-gccbugzilla at metafoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66478

            Bug ID: 66478
           Summary: [constexpr] accepts-invalid with read of non-constant
                    variable as discarded value
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: richard-gccbugzilla at metafoo dot co.uk
  Target Milestone: ---

GCC accepts this ill-formed code:

  void f(int n) { static_assert((+n, true), ""); }

This is invalid because it reads the variable 'n', whose value is not known
within the constant expression.