Mystery with "others"

[gautier_niouzes@hotmail.com]

Hello,

I get an error message with the initialization of variable s4 below, that I don't understand.
The workaround is easy, using the expression for variable s3.
Still, I am curious about the reason.

GNAT GPL 2012, with any of the modes Ada 95, 2005 and 2012, I get:
test_sets.adb:10:21: "others" choice not allowed here
ObjectAda 7.2.2 (Ada 95 only) is more verbose
test_sets.adb: Error: line 10 col 17 LRM:4.3.3(11), An OTHERS is not allowed in this context because there is no applicable_index_constraint, Continuing

So why is Ada unhappy with the expression after "s4 := " ?

---8<----8<----8<----8<--
procedure Test_sets is

  type Enum is (a,b,c,d);

  type Set is array(Enum) of Boolean;

  s1: Set := (a => True, others => False);
  s2: Set := (d => True, others => False);
  s3: Set := s1 or s2;
  s4: Set := s1 or (d => True, others => False); -- <- Line 10

begin
  null;
end;
---8<----8<----8<----8<--
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