Fix check whether classtag can be generated for match types #16708
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The previous check, meant to address scala#15618, tested whether all alternatives of a match type had the same class tag and only then permitted a classtag for the match type. This was the wrong test. It did not work for recursive match types, because it could lead to stack overflow for them. And it did not take into account that match types could be reduced. A better test is to simply declare that match types themselves don't have a stable erasure, just like TypeBounds don't have a stable erasure. If we find an applied type with a match type as definition, we proceed to its translucent superytype, which will try a match type reduction. If that succeeds we produce the classtag of the redux. If not, we end up with an unreduced matchtype and refuse to generate a classtag for it. Fixes scala#16706 Fixes scala#16707
odersky
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The previous check, meant to address #15618, tested whether all alternatives of a match type had the same classtag and only then permitted a classtag for the match type. This was the wrong test. It did not work for recursive match types, because it could lead to stack overflow for them. And it did not take into account that match types could be reduced.
A better test is to simply declare that match types themselves don't have a stable erasure, just like TypeBounds don't have a stable erasure. If we find an applied type with a match type as definition, we proceed to its translucent supertype, which will try a match type reduction. If that succeeds we produce the classtag of the redux. If not, we end up with an unreduced matchtype and refuse to generate a classtag for it.
Fixes #16706
Fixes #16707