From zefram@fysh.org

$ ./perl6 -e 'my Mu $a is default(Mu); say $a'
===SORRY!=== Error while compiling -e
Can't use unknown trait 'is default' in a variable declaration.
at -e​:1
------> my Mu $a is default(Mu)^; say $a
  expecting any of​:
  TypeObject
  default
  dynamic

The "is default" trait *is* known, and works fine with most other values.
Furthermore, Mu is a valid value for this variable, so this declaration
should be semantically permitted.

If a different type constraint is used, that the value Mu doesn't satisfy,
then the incorrect "unknown trait" error is seen rather than a type
mismatch error. The incorrect error does not occur with a declared
default value of Any, Cool, or other such abstract type objects, nor
with Int or other concrete class objects. But it does occur with (Mu
but True), or with a user-defined class declared "is Mu".

-zefram

From @jnthn

On Sat Sep 19 10​:45​:37 2015, zefram@​fysh.org wrote​:

$ ./perl6 -e 'my Mu $a is default(Mu); say $a'
===SORRY!=== Error while compiling -e
Can't use unknown trait 'is default' in a variable declaration.
at -e​:1
------> my Mu $a is default(Mu)^; say $a
expecting any of​:
TypeObject
default
dynamic

The "is default" trait *is* known, and works fine with most other values.
Furthermore, Mu is a valid value for this variable, so this declaration
should be semantically permitted.

If a different type constraint is used, that the value Mu doesn't satisfy,
then the incorrect "unknown trait" error is seen rather than a type
mismatch error. The incorrect error does not occur with a declared
default value of Any, Cool, or other such abstract type objects, nor
with Int or other concrete class objects. But it does occur with (Mu
but True), or with a user-defined class declared "is Mu".

Fixed, and test added in S02-names/is_default.t.

The RT System itself - Status changed from 'new' to 'open'

@jnthn - Status changed from 'open' to 'resolved'