Overridden tasks that use super value are not cached

[rtimush]

If a task overrides one from a super trait and uses the base value, the caching is broken:

// base trait
def foo = T { Seq("base") } 
// subclass
override def foo = T{ super.foo() ++ Seq("override") }

Here is the test case: https://github.com/lihaoyi/mill/compare/task-override

[lihaoyi]

This is probably a consequence of my recent Mirror/Discovered changes. I’ll take a look