Calling .substr with a Callable for $from that creates a Rat dies

[MasterDuke17]

The Problem

[21:29] <colomon> p6: say “5134”.substr(0, */2)
[21:29] <+camelia> rakudo-moar 404f30217: OUTPUT: «This type cannot unbox to a native integer: P6opaque, Rat␤  in block <unit> at <tmp> line 1␤␤»
[21:29] <colomon> p6: say “5134”.substr(0, * div 2)
[21:29] <+camelia> rakudo-moar 404f30217: OUTPUT: «51␤»
[21:34] <colomon> documentation ( https://docs.perl6.org/routine/substr ) seems off, plus requiring an Int instead of a Rat seems needlessly picky

.substr can take a Callable for several of its parameters, but */2 dies, while * div 2 is ok.

Expected Behavior

If Rats should be allowed, then say “5134”.substr(0, */2) should output "51".

For these multis for .substr https://github.com/rakudo/rakudo/blob/master/src/core/Str.pm6#L2719-L2762,
a potential fix is converting lines like this my int $from = (start)(nqp::chars($!value)) to my int $from = ((start)(nqp::chars($!value))).Int. I tried this and it correctly fixed calling with */2, but I didn't run a spectest.

Actual Behavior

Dies with This type cannot unbox to a native integer: P6opaque, Rat in block <unit> at -e line 1

Steps to Reproduce

perl6 -e 'say “5134”.substr(*/2)'

Environment

• Operating system: Kubuntu 17.10 Linux hermes 4.13.0-37-generic #42-Ubuntu SMP Wed Mar 7 14:13:23 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
• Compiler version (perl6 -v): This is Rakudo version 2018.03-206-g0a08f1d0a built on MoarVM version 2018.03-99-g4234ab56b