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milessabin/gist:5611702

Created May 20, 2013 11:25
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Since when was "<:" an alias for "extends" ... not that I'm complaining, mind ;-)
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gistfile1.scala
scala> trait Foo ; trait Bar <: Foo
defined trait Foo
defined trait Bar
scala> implicitly[Bar <:< Foo]
res2: <:<[Bar,Foo] = <function1>
@oxbowlakes
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But (at least in 2.10.1):

scala> class Baz <: Bar
<console>:1: error: classes are not allowed to be virtual
       class Baz <: Bar
                 ^

@4lex1v
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4lex1v commented May 20, 2013

Also throws an error for abstract classes

@xuwei-k
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xuwei-k commented May 20, 2013

2.9.2 and 2.8.2 also works!

but 2.7.7

scala> trait Foo ; trait Bar <: Foo
<console>:1: error: ';' expected but '<:' found.
       trait Foo ; trait Bar <: Foo
                             ^

@milessabin
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@larsrh discovered this in scalac,

if (in.token == EXTENDS ||
  settings.YvirtClasses && (mods hasFlag Flags.TRAIT) && in.token == SUBTYPE) { ...

So it looks like it's a relic of the virtual classes experiment.

@soc
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soc commented May 20, 2013

In general, it seems to be nice to have either <: or extends, not both.

@milessabin
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This would appear to be the offending commit (search for YvirtClasses) scala/scala@4253124 ... calling @paulp ...

@milessabin
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See also http://stackoverflow.com/questions/2123663/what-does-trait-a-b-mean

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