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Bug introduced in 10.0.0 and fixed in 10.1.0.

While answering Pattern match for nested Association I came across behavior of MemberQ and FreeQ in 10.0.1 that I believe is incorrect. The option Heads -> False does not work properly:

expr1 = foo[bar, baz];
expr2 = <|1 -> a, 2 -> b|>;

MemberQ[expr1, _?Print, Heads -> False];

bar

baz

MemberQ[expr2, _?Print, Heads -> False];

Association

a

b

FreeQ[expr1, _?Print, Heads -> False];

bar

baz

foo[bar,baz]

FreeQ[expr2, _?Print, Heads -> False];

Association

a

b

<|1->a,2->b|>

Observe that in both cases Association is checked against the pattern even though foo is not.

Sjoerd wrote:

I believe the error is not the printing of Association but that it prints anything at all. Association is atomic and it should have no (visible) members. Compare with expr3=Graph[{1->2}].

The 10.0.1 documentation says:

MemberQ works on associations, testing values but not keys.

FreeQ works on associations, testing values but not keys.

Therefore this overloading is documented and I believe options should work propertly.

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    $\begingroup$ I believe the error is not the printing of Association but that it prints anything at all. Association is atomic and it should have no (visible) members. Compare with expr3=Graph[{1->2}]. $\endgroup$ – Sjoerd C. de Vries Oct 8 '14 at 19:06
  • $\begingroup$ @Sjoerd Please see update. $\endgroup$ – Mr.Wizard Oct 8 '14 at 23:51
  • $\begingroup$ I see that this Question has six votes. Shall I read that to mean that people agree that this appears to be a bug, and tag it as such? $\endgroup$ – Mr.Wizard Oct 9 '14 at 12:23
  • $\begingroup$ AFAIC yes, and you were right with your update concerning the documentation. I hadn't seen the updates to these parts of the documenation. $\endgroup$ – Sjoerd C. de Vries Oct 9 '14 at 13:14
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    $\begingroup$ Reported and confirmed as a bug. $\endgroup$ – Daniel Lichtblau Nov 26 '14 at 17:54
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Daniel Lichtblau confirmed that this was a bug. It has been corrected in version 10.1.0.

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