The following program fails with an internal compiler error:

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enum Colors:
ORANGE
BLACK
MAGENTA

class ColorAttribute(System.Attribute):
def constructor(c as Colors):
pass

class Foo:
[Color(Colors.ORANGE)]
public bar as int

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The error is as follows:

attrtest.boo(12,10): BCE0055: Internal compiler error: Type must be a type provided by the runtime.
Parameter name: enumType.

Changing the "Colors" type in the attribute constructor to a plain object is a workaround for this problem.