Don't trust case class extractors with explicit type arguments #15669
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scala#6551 introduced an exception where type tests of parameterized case classes were not flagged as "type test cannot be checked at runtime". The idea was that the parameters would be inferred by GADT reasoning and would therefore be correct. But with scala#15356 we now also allow explicit type arguments in extractors. If these are given we cannot guarantee that a type test for a case class in an unapply will always succeed. So we need an unchecked warning.
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I disagree this being a compile time error. Following the principle of least surprise, these three cases should behave equally, since they constrain the matched type equally: case Composite[Int](v) =>
case Composite(v: Int) =>
case Composite[Int](v: Int) =>But this is a good stop-gap solution however. |
I am afraid the theory of GADTs is a minefield, and full of surprises. So least surprise would be nice, but unattainable. And it's not just theory either. Every misstep is backed by a test throwing a ClasscastException. |
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My reading is that it's not a compile error, it's a made-fatal unchecked warning, no? |
dwijnand
approved these changes
Jul 13, 2022
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#6551 introduced an exception where type tests of parameterized case classes were not flagged
as "type test cannot be checked at runtime". The idea was that the parameters would be inferred
by GADT reasoning and would therefore be correct.
But with #15356 we now also allow explicit type arguments in extractors. If these are given
we cannot guarantee that a type test for a case class in an unapply will always succeed.
So we need an unchecked warning.
Fixes #15662