Fix #401: Avoid asSeenFrom for types/bounds that don't depend on the prefix. #404
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…end on the prefix. This mostly acts as a fast path. However, for the particular case of static recursive match types, it more critically cuts some infinite recursion. Normally, when a match type refers to itself, it would do so through a `ThisType`. Computing `asSeenFrom` for a `ThisType` prefix already cuts the infinite recursion because it is, by construction, a no-op. However, for static recursive match types, the compiler makes them refer to themselves through their public, static prefix. This caused the infinite recursion observed in scalacenter#401. To avoid the issue, we use another shortcut to `asSeenFrom`: if the type we map over does not contain any `ThisType`, we also know it will be a no-op. We store that as a lazy val `isPrefixDependent` of `TermSymbol` and `TypeMemberSymbol`, so that they can take a short cut in `{type,bounds}AsSeenFrom`.
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This mostly acts as a fast path. However, for the particular case of static recursive match types, it more critically cuts some infinite recursion.
Normally, when a match type refers to itself, it would do so through a
ThisType. ComputingasSeenFromfor aThisTypeprefix already cuts the infinite recursion because it is, by construction, a no-op.However, for static recursive match types, the compiler makes them refer to themselves through their public, static prefix. This caused the infinite recursion observed in #401.
To avoid the issue, we use another shortcut to
asSeenFrom: if the type we map over does not contain anyThisType, we also know it will be a no-op. We store that as a lazy valisPrefixDependentofTermSymbolandTypeMemberSymbol, so that they can take a short cut in{type,bounds}AsSeenFrom.