Backport "Fix signature computation and caching involving type variables " to LTS #19107
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Its meaning will be expanded in the next commit. [Cherry-picked 84bc1bd]
During typechecking, we sometimes need to compute the signature of a type involving uninstantiated type variables or wildcards (in particular, because signature matching is doen as part of subtype checking for methods), this is handled with `tpnme.Uninstantiated` which is handled specially in `Signature`. Before this commit, `sigName` only checked for wildcards and type variables at the top-level of the type, even though nested types can still have an impact on type erasure, in particular when they appear as part of: - an intersection - a union - an underlying type of a derived value class - the element type of an array type - an element type of a tuple (... *: X *: ...) We keep track of all these situations by returning `null` in `TypeErasure#apply` when encountering a wildcard or uninstantiated type variable, which we then propage upwards to the `sigName` call. This propagation only happens for `sigName` calls (where `inSigName` is true), otherwise we throw an assertion since erasure shouldn't normally be computed for underdefined types. [Cherry-picked c21133f]
Signatures should not be cached if they may change. The existing `isUnderDefined` check is not enough to guarantee this because the signature might have been computed based on a type variable instantiated in a TyperState which was subsequently retracted. To guard against this, we now rely on `Type#isProvisional`. This is a stricter check than needed since the provisional part of the type does not always have an impact on its signature, but in practice this is fine: when compiling Dotty, signature cache misses increased by less than 2%. [Cherry-picked 3729960]
I'm afraid we'll have to get rid of it if we want our caches to be correct, but I don't have the time to look into it right now. [Cherry-picked 7de3663]
As requested in the review, this is slightly shorter but means we have to be very careful to not accidentally forget to propagate a WildcardType to the top-level when computing signatures. [Cherry-picked 4499bc0]
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No regressions detected in the community build up to lts-16941. |
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Backports #18092 to the LTS branch.
PR submitted by the release tooling.
[skip ci]