Try to be more subtle when inferring type parameters of class parents #16896
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The previous scheme was basically like this: 1. If the parent type is an application, infer type parameters from the term parameters during completion of the class. 2. If the parent type is an Ident or Select and it appears in an anonymous class, infer it from the expected type. It could happen in the second case that we infer Nothing since an expected type was missing, but if the parent type had been an application, we would have inferred something else from implicit arguments to the parent constructor. This is a case handled by Scala 2, but not yet by Dotty. To deal with this we have to perform a complicated dance: - Try step (2) above, but back out if the inferred parent type has Nothing as a type argument. - During completion, if the inferred parent has missing type arguments, convert the parent type to an application with () arguments and try that instead. I normally would have thought this is too much sophistry but there are valid use cases that Scala 2 supports and it would be good if we get to parity for these.
dwijnand
approved these changes
Feb 13, 2023
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The previous scheme was basically like this:
It could happen in the second case that we infer Nothing since an expected type was missing, but if the parent type had been an application, we would have inferred something else from implicit arguments to the parent constructor. This is a case handled by Scala 2, but not yet by Dotty.
To deal with this we have to perform a complicated dance:
I normally would have thought this is too much sophistry but there are valid use cases that Scala 2 supports and it would be good if we get to parity for these.
Fixes #16778