bugs - Pattern matching with List and Optional

Bug introduced in 8.0.4 or earlier and persists through 11.1.1

Thank you for bringing this to our attention. I have filed a report with our developers.

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Why does this

{42.`}~MatchQ~(x0 : _List)
{42.`}~MatchQ~(x0 : {___})

{42.`}~MatchQ~(x0 : _List : {1.})
{42.`}~MatchQ~(x0 : {___} : {1.})

give

True

True

True

False

Instead of all true?

Answer

This bug is fixed in our next release. Note, however, that, that the use of Optional generates a warning now, as the optional value could never possible be used in such pattern:

In[14]:= {42.`}~MatchQ~(x0 : {___} : {1.})
During evaluation of In[14]:= Optional::optloose: Optional pattern 
x0:{___}:{1.} has no enclosing pattern so no optional value can be matched.
Out[14]= True