[perl #129278] [BUG] Can't assign to `int` private attribute using signature

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# New Ticket Created by  Zoffix Znet 
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#Doesn't work:
<TestNinja> m: class { has int $!n = 0; method x ($!n) {} }.new.x: 42
<camelia> rakudo-moar 2c95f7: OUTPUT«Cannot modify an immutable int␤  in method 
x at <tmp> line 1␤  in block <unit> at <tmp> line 1␤␤»

# But does, if we use a temp var:
<TestNinja> m: class { has int $!n = 0; method x ($n) {$!n = $n} }.new.x: 42
<camelia> rakudo-moar 2c95f7: ( no output )