fix(type-checker): enforce invariant generic arguments

[chess10kp]

Closes #8580
Fixes #8550

Summary

• Enforce declared generic variance when source and destination share the same class.
• Require invariant type arguments to be assignable in both directions.
• Apply the same bidirectional invariant check to generic base-class arguments.
• Add checker coverage for list[int] to list[float] and list[Dog] to list[Animal].

Root cause

_assign_class compared same-class generic arguments only as source -> destination. The generic checking implementation introduced this behavior in 225a2c9176 (#5159), so the defect was pre-existing on upstream/main.

Verification

• cd jac && JAC_TEST_JOBS=0 jac test tests/compiler/passes/test_typevar.jac — 10 passed.
• Focused reproduction without append now reports two expected E1053 errors.

[christianwilkins]

Reviewed 2ebe6bb66a53b1235d3267c881047459dc2c0f20, including the full patch, both generic comparison paths, parser callers, existing discussion and current-head CI. Two remaining soundness holes are attached inline: object arguments bypass invariance, and union arguments fall back to a one-way comparison. These are incomplete cases of the defect this patch is intended to fix; the original two regression cases do pass.

Independent validation on #9010 head 725f41cfe: jac test jac/tests/compiler/passes/test_typevar.jac passes all 11 tests. Two additional jac check probes both incorrectly pass: list[Dog] passed to list[object] (whose body appends a string), and list[Dog | str] passed to list[Animal | str], with Dog(Animal). I compared the complete trees of #9007 and #9010: their compiler, parser, tests and other source are byte-identical; only the release-note filename differs. Thus this is one local validation run of their shared implementation, not two independent runs. git diff --check passes; no code changes were made during review.

The latest follow-up 2ebe6bb66 narrows the earlier builtin exemptions, but retains the object exception and non-ClassType fallback. Both PRs are conflict-free. Current-head CI is red because jac fmt --check --lintfix fails for type_evaluator.impl.jac and test_typevar.jac; the job logs include the required formatting patch.

Status: changes requested. Author work: enforce invariant assignment for these concrete argument forms in both same-class and generic-base paths, add the negative cases, apply formatting, and rerun the checks. This overlaps #9010 exactly, so coordinate which PR should land and close the duplicate after the chosen fix is accepted. No merge or closure was performed in this review.

[christianwilkins]

[P2] Do not exempt object from mutable-container invariance. On this implementation, def accept_objects(values: list[object]) -> None { values.append("not a dog"); } accepts a dogs: list[Dog] argument without any checker error. object is a concrete supertype, so the callee can insert values that violate the caller's element type. This branch skips the reverse assignment that would reject it. Preserve gradual Any/Unknown behavior separately and enforce both directions here and in the matching generic-base branch.

[christianwilkins]

[P2] Apply the reverse check to union arguments too. The invariant check only becomes bidirectional when both arguments are ClassType. For Dog(Animal), def accept_union(values: list[Animal | str]) -> None {} still accepts dogs: list[Dog | str] with no checker error in my probe. Either argument being a UnionType reaches this one-way fallback, retaining the same mutable-container covariance hole. Add this negative case and use the invariant rule for concrete non-ClassType arguments in both comparison paths.

[greptile-apps]

Greptile Summary

The PR enforces declared variance while comparing generic class arguments and adds regression coverage for invariant mutable containers.

• Invariant generic arguments are checked in both assignment directions.
• Covariant and contravariant parameters retain their directional checks.
• The same variance handling applies when matching generic base classes.
• Supporting compiler sources were adjusted to satisfy the stricter type checker.
• Tests verify that list[int] cannot substitute for list[float] and list[Dog] cannot substitute for list[Animal].

Confidence Score: 5/5

The PR appears safe to merge; no new issue was introduced after the previous review, and the earlier finding was manually resolved.

The current review delta contains no changes after the previous review SHA, so there are no new regressions to report. The sole previous finding was manually resolved without explanation and therefore is not outstanding.

Important Files Changed

Filename	Overview
jac/jaclang/compiler/types/type_evaluator.impl/type_evaluator.impl.jac	Enforces covariant, contravariant, and bidirectional invariant argument checks for same-class and generic base-class assignment.
jac/tests/compiler/passes/fixtures/checker/checker_invariant_generics.jac	Provides representative numeric and class-hierarchy substitutions that invariant mutable containers must reject.
jac/tests/compiler/passes/test_typevar.jac	Verifies that both unsound invariant substitutions emit assignment diagnostics.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Compare source and destination class types] --> B{Same class or generic base?}
    B -->|No| C[Continue inheritance matching]
    B -->|Yes| D{Declared parameter variance}
    D -->|Covariant| E[Check source argument → destination argument]
    D -->|Contravariant| F[Check destination argument → source argument]
    D -->|Invariant| G[Check both directions]
    E --> H[Accept only if check succeeds]
    F --> H
    G --> H

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_{Reviews (3): Last reviewed commit: "ci: retrigger jac-check type sweep" | Re-trigger Greptile}