GROOVY-12166 backport: STC: scope statement-level instanceof narrowing to the …

[paulk-asert]

…member being visited

Temporary type information from instanceof is keyed by the target variable — for a field access, the shared FieldNode. if/loops/ternaries and expression statements bracket their own frames, but a narrowing recorded at statement level (an explicit return field instanceof Sub, or an assert field instanceof Sub, whose propagation to subsequent statements is intentional) lands in the enclosing frame, which was the frame pushed for the whole class visit. Keyed by the shared FieldNode, it then applied to every member visited afterwards: the checker stamped the narrowed type on unrelated field reads and the static compiler emitted a spurious checkcast, throwing ClassCastException at runtime whenever the field held a different subtype.

Members now push their own frame (methods and constructors, property and field initializers, object initializer blocks), so statement-level narrowing still flows within a member but can never survive into another; narrowing has no meaning past the member boundary. Intra-member flow typing (assert, if-branch) is unchanged.

The checker-side leak predates Groovy 5 but was masked by codegen: StaticTypesTypeChooser resolved types from the declared target first until GROOVY-11375 (5.0.0-alpha-9) gave the expression-stamped inferred type priority, exposing the stale narrowing as a checkcast. That change is correct; the fix belongs here.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.2789%. Comparing base (514e66f) to head (8218af1).

Additional details and impacted files

@@                  Coverage Diff                   @@
##             GROOVY_5_1_X      #2797        +/-   ##
======================================================
+ Coverage         67.2715%   67.2789%   +0.0074%     
  Complexity          29631      29631                
======================================================
  Files                1383       1383                
  Lines              117109     117117         +8     
  Branches            20596      20596                
======================================================
+ Hits                78781      78795        +14     
+ Misses              31777      31774         -3     
+ Partials             6551       6548         -3     

Files with missing lines	Coverage Δ
...roovy/transform/stc/StaticTypeCheckingVisitor.java	87.4009% <100.0000%> (+0.0276%)	⬆️

... and 2 files with indirect coverage changes

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