Proposed resolution:

This wording is relative to N4659.

1. Modify [mem.res.global] as follows:

   memory_resource* set_default_resource(memory_resource* r) noexcept;
   

   -4- Effects: If r is non-null, sets the value of the default memory resource pointer to r, otherwise sets the default memory resource pointer to new_delete_resource().

   -5- Postconditions: get_default_resource() == r.

   -6- Returns: The previous value of the default memory resource pointer.

   -7- Remarks: […]

[ 2017-06-15, Moved to Tentatively Ready after 6 positive votes on c++std-lib ]

[ 2017-06-13, Casey Carter revises proposed wording ]

I suggest to strike the Postconditions paragraph ([mem.res.global]/5) completely, as in LWG 2522

[ 2017-05-13, Tim comments ]

This is the same issue as LWG 2522, which just missed the Fundamentals TS working draft used for the merge. A similar resolution (simply striking the Postconditions: paragraph) might be better.

Previous resolution [SUPERSEDED]:

This wording is relative to N4659.

1. Modify [mem.res.global] as follows:

   memory_resource* set_default_resource(memory_resource* r) noexcept;
   

   -4- Effects: If r is non-null, sets the value of the default memory resource pointer to r, otherwise sets the default memory resource pointer to new_delete_resource().

   -5- Postconditions: If r is non-null, get_default_resource() == r. Otherwise, get_default_resource() == new_delete_resource().

   -6- Returns: The previous value of the default memory resource pointer.

   -7- Remarks: […]

The specification of set_default_resource in N4659 [mem.res.global] reads:

memory_resource* set_default_resource(memory_resource* r) noexcept;

-4- Effects: If r is non-null, sets the value of the default memory resource pointer to r, otherwise sets the default memory resource pointer to new_delete_resource().

-5- Postconditions: get_default_resource() == r.

-6- Returns: The previous value of the default memory resource pointer.

-7- Remarks: […]

It is apparent that the effects specified in para 4 cannot — and indeed should not — achieve the postcondition specified in para 5 when r is null.