From Sema::CheckForConstantInitializer:
  // FIXME: Need strict checking.  In C89, we need to check for
  // any assignment, increment, decrement, function-calls, or
  // commas outside of a sizeof.  In C99, it's the same list,
  // except that the aforementioned are allowed in unevaluated
  // expressions.  Everything else falls under the
  // "may accept other forms of constant expressions" exception.
  // (We never end up here for C++, so the constant expression
  // rules there don't matter.)

Filing here for completeness.  This is relatively low-priority because it's a non-trivial amount of work, and it's a warning nobody would care about outside of -pedantic mode.