[perl #77164] [BUG] Cannot always match signature when default value is a sub

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# New Ticket Created by  Patrick Abi Salloum 
# Please include the string:  [perl #77164]
# in the subject line of all future correspondence about this issue. 
# <URL: http://rt.perl.org/rt3/Ticket/Display.html?id=77164 >


<patrickas> rakudo: sub foo ( $f = rand ) { say $f; }; say
Capture.new() ~~ &foo.signature;  #does not match
<p6eval> rakudo c1e19a: OUTPUT«0␤»

<patrickas> rakudo: sub foo ( $f = 0.461541885367723 ) { say $f;  };
say Capture.new() ~~ &foo.signature;  #matches
<p6eval> rakudo c1e19a: OUTPUT«1␤»

<patrickas> rakudo: sub foo ( $f = substr('string',0) ) { say $f; };
say Capture.new() ~~ &foo.signature; #matches
<p6eval> rakudo c1e19a: OUTPUT«1␤»