From @zoffixznet

This produces a confusing error​:

<ZoffixW> m​: class Foo is Int{}
<camelia> rakudo-moar 42a583​: OUTPUT«===SORRY!=== Error while compiling /tmp/poMXu0zSBD␤Cannot call trait_mod​:<is>(Foo, Int, Hash); none of these signatures match​:␤ (Mu​:U $child, Mu​:U $parent)␤ (Mu​:U $child, :$DEPRECATED!)␤ (Mu​:U $type, :$rw!)␤ (Mu​:U $type, :$nativ…»

Perhaps, the same thing that's done for subs can be extended for classes too​:
<ZoffixW> m​: sub foo is export{}
<camelia> rakudo-moar 42a583​: OUTPUT«===SORRY!===␤Trait 'is export' needs whitespace before block␤at /tmp/5tO_UVrVAD​:1␤------> sub foo is export{}⏏<EOL>␤Missing block (apparently claimed by 'is export')␤at /tmp/5tO_UVrVAD​:1␤------> sub foo is export{}⏏<EOL>…»
<TimToady> maybe that's what we're thinking of

Relevant IRC conversation​: http://irclog.perlgeek.de/perl6/2016-01-11#i_11869850

The error message looks awesome now:

$ ./rakudo-m -e 'class Foo is Int{}'
===SORRY!=== Error while compiling -e
Parent class specification is probably missing some whitespace.  Found
'Foo is Int{ ...', which tries to specify a parent with a 'Hash'.  You
probably meant 'Foo is Int { ...'.
at -e:1

The problem was also mentioned in rakudo/rakudo#2052 and was fixed/improved with rakudo/rakudo@21eb168ca4, rakudo/rakudo@1956df255207 and later on rakudo/rakudo@98c7e512a6.

IMHO it would be nice to have a test for Rakudo, so I'm leaving this issue open for now.