[tlively]

Default-initialize PrincipalType::unreachable so the Signature constructor does not leave it uninitialized, and grow AssignmentClasses sets up to max(i, *other) before looking up classes.getRoot(*other).

Implement the remaining join cases for VarNullability, VarExactness, VarSharedness, VarHeapType (including mixed abstract/defined heap types and defined heap types with basic LUBs), and VarRef/Type, as well as erasing leading unreachable elements at the bottom of results in PrincipalType::compose.

[gemini-code-assist]

Code Review

This pull request implements support for joining abstract and defined heap types, refactors nullability, exactness, and sharedness join operations, and improves the handling of unreachable types during principal type composition. It also adds comprehensive unit tests to verify these changes. As there are no review comments, I have no feedback to provide.

[tlively]

Here is the plan:

1. Finish implementing composition of principal types (this PR)
2. Fix bugs in child-typer.h and use it as part of PrincipalType::fromExpression
   1. At this point, progress on Outlining will be unblocked
3. Add a ConcreteChildTyper utility that computes the upper bounds on the types of children from an upper bound on the type of the parent.
   1. For efficiency, this will not actually use PrincipalType because the forward principal types property lets us do this kind of analysis without having to worry about any kind of polymorphism.
   2. There will be a test-only pass that checks that ConcreteChildTyper and an analysis using PrincipalType give the same results. We will run this in the fuzzer.
4. Use ConcreteChildTyper in TypeGeneralizing, StripRefiningCasts, etc.