relax double parens in expression_function_declaration?

[Stephen Leake <stephen_leake@stephe-leake.org>]

Recent GNAT allows this:

   type Result is record
      A : Integer;
      B : Integer;
   end record;

   function Key return Result is
     (A => 1,
      B => 2);

However, I can't find a rule in the ARM that allows it.
ARM 6.8 says:

   expression_function_declaration ::=
        [overriding_indicator]
        function_specification is
            (expression)
            [aspect_specification];

This requires double parens:

   function Key return Result is
     ((A => 1,
       B => 2));

ARM 4.5.7(7) gives permission to remove the double parens for conditional_expression; is there a similar rule or AI for aggregates?