[perl #128879] [LTA] “is default” trait is compile-time, but no warning is printed when I do stupid things (is default($x))

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# New Ticket Created by  Aleks-Daniel Jakimenko-Aleksejev 
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Code:
my $x = 42;
my @a is default($x);
say @a[1]

Result:
(Any)


It is doc-ed correctly, so one might argue that the user should've known
better. But it is very LTA, and if there is a way to warn the user (or why
warn, let's make it a compile-time error?) then we should definitely do it.