Type mismatch on abstract type used in pattern matching

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The first case is unsound because you underestimate the variety of types in Scala type system. It would make sense if, when we took case i:Int branch we knew T was Int, or at least a supertype of Int. But it doesn’t have to be! E.g. it could be 42.type or a tagged type.

There’s no such problem in the second case, because from IntExpr <: Expr[T], the compiler does know T must be exactly Int.

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