Proposed resolution:

This wording is relative to N4296.

1. Change [allocator.requirements], Table 28 — "Allocator requirements", as indicated:

   Table 28 — Allocator requirements

   Expression	Return type	Assertion/note pre-/post-condition	Default
   …
   a.max_size()	X::size_type	the largest value that can meaningfully be passed to X::allocate()	numeric_limits<size_type>::max()/sizeof(value_type)
   …

2. Change [allocator.traits.members]/p7 as indicated:

   static size_type max_size(const Alloc& a) noexcept;
   

   Returns: a.max_size() if that expression is well-formed; otherwise, numeric_limits<size_type>::max()/sizeof(value_type).

[ 2015-05, Lenexa ]

Marshall: Is this the right solution?
PJP: I think it's gilding the lily.
STL: I think this is right, and it doesn't interact with the incomplete container stuff because it's in a member function.
Marshall: Objections to this?
STL: Spaces around binary operators.
Hwrd: It's completely wrong without spaces.
Marshall: All in favor of Ready?
Lots.

Table 28 — "Allocator requirements" says that default behavior for a.max_size() is numeric_limits<size_type>::max(). And this is consistent with the matching statement for allocator_traits in [allocator.traits.members]/p7:

static size_type max_size(const Alloc& a) noexcept;

Returns: a.max_size() if that expression is well-formed; otherwise, numeric_limits<size_type>::max().

However, when allocating memory, an allocator must allocate n*sizeof(value_type) bytes, for example:

value_type*
allocate(std::size_t n)
{
  return static_cast<value_type*>(::operator new (n * sizeof(value_type)));
}

When n == numeric_limits<size_type>::max(), n * sizeof(value_type) is guaranteed to overflow except when sizeof(value_type) == 1.

A more useful default would be numeric_limits<size_type>::max() / sizeof(value_type).