Lift all non trivial prefixes for default parameters #19739
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nicolasstucki
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odersky
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Feb 27, 2024
| * unless `pre` is idempotent reference, a `this` reference, a literal value, or a or the prefix of an `init` (`New` tree). | ||
| * | ||
| * Note that default arguments will refer to the prefix, we do not want | ||
| * to re-evaluate a complex expression each time we access a getter. | ||
| */ | ||
| def liftPrefix(defs: mutable.ListBuffer[Tree], tree: Tree)(using Context): Tree = |
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Can we get the old behavior when there are no default arguments? We'd need a boolean passed to liftPrefix indicating whether the enclosing method has the HasDefaultParams flag.
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I split this definition into liftNonIdempotentPrefix and liftPrefix instead. Compining them made the documentation to convoluted. HasDefaultParams is used to choose which of those methods to call.
Checking if the prefix is pure is not enough to know if we need to list the prefix. In the case of default parameters, the prefix tree might be used several times to compute the default values. This expression should only be computed once and therefore it should be lifted if there is some computation/allocation involved. Furthermore, if the prefix contains a local definition, it must be lifted to avoid duplicating the definition. A similar situation could happen with dependent default parameters. This currently works as expected. Fixes scala#15315
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Checking if the prefix is pure is not enough to know if we need to list the prefix. In the case of default parameters, the prefix tree might be used several times to compute the default values. This expression should only be computed once and therefore it should be lifted if there is some computation/allocation involved. Furthermore, if the prefix contains a local definition, it must be lifted to avoid duplicating the definition.
A similar situation could happen with dependent default parameters. This currently works as expected.
Fixes #15315