Avoid box failures and better diagnosis if they come back #24546
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odersky
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odersky
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Linyxus
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Nov 27, 2025
| val (boxed, unboxed) = | ||
| if cs.tp1.isBoxedCapturing then (cs.tp1, cs.tp2) else (cs.tp2, cs.tp1) | ||
| i"""${cs.tp1} does not conform to ${cs.tp2} because $boxed is boxed but $unboxed is not. | ||
| |This could be an error in the capture checker implementation.""" |
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Not sure about this error note. It feels to me a bit ambiguous: should the user open an issue ticket when they see this?
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We are not sure whether it's an error or not, so I refrained from giving clear guidance to users.
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In fact eta.scala still shows the same error.
At least we don't get
Note that cap is not included in {}
anymore. To make this even better we'd have to trace back a box
failure to the original types we were trying to compare. Right now
the box failure error notes are too disconnected from the rest. But
they become clearer when one compiles with -explain.
The neg tests are now pos tests. I kept the special error message for better diagnosis if the mismatch arises elsewhere (it does in eta.scala).
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The first commit gives a better error message for box failures. At least we don't get
anymore. To make this even better we'd have to trace back a box failure to the original types we were trying to compare. Right now the box failure error notes are too disconnected from the rest. But they become clearer when one compiles with -explain.
The second commit fixes the underlying problem. The neg tests are now pos tests. I kept the special error message for better diagnosis if the mismatch arises elsewhere. The mismatch still persists in eta.scala. We should investigate
whether that's legit or not.
Fixes #24543