From @zoffixznet

# I have the fix; filing for records

The .subst/.subst-mutate methods slurp several of their options into a hash and then pass those
along to Str.match call. The logic that handles the result of that call, however, does not
pay attention to Failures. Specifically, failures resulting from incorrect values provided for
the named arguments that get passed to .match​:

m​: say 'foo'.subst​: /\w/, '', :x{2}
rakudo-moar 222d16​: OUTPUT«foo␤»
m​: say 'foo'.subst​: /\w/, '', :th(2.5)
rakudo-moar 222d16​: OUTPUT«foo␤»
m​: say ($ = 'foo').subst-mutate​: /\w/, '', :th(2.5)
rakudo-moar 222d16​: OUTPUT«Nil␤»

Thus, the user never receives any indication they're using the wrong arguments and the code behaves as the match simply fails.

From @zoffixznet

Fixed in rakudo/rakudo@6ef4cdf543
Tests added in Raku/roast@925cf4bd8b

On Tue Sep 27 11​:39​:00 2016, cpan@​zoffix.com wrote​:

# I have the fix; filing for records

The .subst/.subst-mutate methods slurp several of their options into a
hash and then pass those
along to Str.match call. The logic that handles the result of that
call, however, does not
pay attention to Failures. Specifically, failures resulting from
incorrect values provided for
the named arguments that get passed to .match​:

m​: say 'foo'.subst​: /\w/, '', :x{2}
rakudo-moar 222d16​: OUTPUT«foo␤»
m​: say 'foo'.subst​: /\w/, '', :th(2.5)
rakudo-moar 222d16​: OUTPUT«foo␤»
m​: say ($ = 'foo').subst-mutate​: /\w/, '', :th(2.5)
rakudo-moar 222d16​: OUTPUT«Nil␤»

Thus, the user never receives any indication they're using the wrong
arguments and the code behaves as the match simply fails.

@zoffixznet - Status changed from 'new' to 'resolved'