ReplaceList Except Alternatives bug

Question

Bug introduced in 11.3 or earlier, persisting through 12.0 or later and fixed in 12.3.1 or earlier

[CASE:4352435] and [CASE:4354378]

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In Mathematica 11.3 and 12.0,

ReplaceList[{2,2},{u_,Except[u_|1]}:>True]

incorrectly returns

{True}

In earlier versions (10.1, 10.3, 11.0, 11.1) executing the above code causes the kernel to hang, requiring it to be quit, so this appears never to have worked correctly. See also the Answer below.

In addition, in the earlier versions, the following also hangs:

ReplaceList[{2, 3}, {u_, Except[1 | u_] ..} :> True]

Using Replace instead of ReplaceList is fine; SequenceCases behaves incorrectly like ReplaceList.

The issue is also seen with orderless matches (which presumably are represented by alternatives internally); for example,

 ReplaceList[{2 x, 2 x}, {2 u_, Except[2 u_]} :> True]

incorrectly returns

{True}

in 11.3 and 12.0.

-- The above has been revised as a result of feedback and further testing.

Answer 1

Update. The bug is confirmed by the tech support: [CASE:4352435].

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For

ReplaceList[{2, 3}, {u_, Except[1 | u_] ..} :> True]

Mathematica 8.0.4 prints error:

Except::named: Named pattern variables are not allowed in the first argument of Except[1|u_].

and returns {}, but versions 11.3 and 12.0 both return {True} without any messages.

What seems to be a real bug in the latest versions is that each of the following two lines also returns {True}:

ReplaceList[{2, 2}, {u_, Except[u_ | 1]} :> True]    
SequenceCases[{2, 2}, {u_, Except[u_ | 1]} :> True]

while the pattern doen't match:

MatchQ[{2, 2}, {u_, Except[u_ | 1]}]

False

Replace works as expected though:

Replace[{2, 2}, {u_, Except[u_ | 1]} :> True]
Replace[{2, 3}, {u_, Except[u_ | 1]} :> True]

{2, 2}

True