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Bug introduced in 10.2, and persisting through 11.1

Bug resolved in 12.0

This is strange:

ClearAll[x, y, a, b];
a[{{_Integer, _Integer, _Integer | PatternSequence[]} ..}] := c;
(*this is a match:*)
a[{{1, 1}}]
(*this too:*)
a[{{1, 1, 1}}]

b = CoordinateBoundsArray /@ {{{1, 1}, {1, 1}}};

x = First@b;
y = {{{1, 1}}};

x === y(*True*)

a /@ x(*{a[{{1,1}}]}*)(* this does not finish evaluating?!*)
a /@ y(*{c}*)

If[Not[
  a /@ x === a /@ y(*False*)
  ], "something is wrong"]

Everything works fine if I change a to 

a[{{_Integer, _Integer} ..}] := c;

or if assign the result of CoordinateBoundsArray /@ {{{1, 1}, {1, 1}}} to b

b = {{{{1, 1}}}};

Can someone confirm this?

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  • $\begingroup$ I get the same answer both in V11.0.0 and in V10.4. I can't see any explanation. $\endgroup$ – mikado Aug 25 '16 at 20:11
  • 12
    $\begingroup$ It's a bug in pattern matching for packed arrays. I have filed a report. $\endgroup$ – ilian Aug 25 '16 at 20:18
  • $\begingroup$ Behavior reproduced in 10.3 $\endgroup$ – QuantumDot Sep 17 '16 at 22:30
  • $\begingroup$ I do not reproduce the problem with versions 11.3 and 12.0. $\endgroup$ – Alexey Popkov Oct 24 '19 at 15:58
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Not a proper answer, but we can see that there is a significant internal difference between the two:

Needs["Developer`"]

PackedArrayQ[x]
(* True *)

PackedArrayQ[y]
(* False *)
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  • 1
    $\begingroup$ And I was thinking of some even more fancy optimizations in the interpreter. Nice find! $\endgroup$ – masterxilo Aug 25 '16 at 20:22

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