Fix type parameter nullability matching for covariant types

[utafrali]

Fixes #1528

There's a regression in type parameter nullability matching for covariant generics. CheckIdenticalNullabilityVisitor was being too strict, requiring identical nullability on both sides even when the type system allows for more flexibility.

Added a covariant mode to the visitor that handles this correctly. In covariant positions, RHS can be @nullable while LHS isn't (but not the reverse). Updated the comparison logic for both type arguments and array components.

Tests added to cover this scenario.

Summary by CodeRabbit

• Bug Fixes

  • Improved nullability checking for generic method reference parameters to use covariant semantics. The checker now correctly handles cases where a method reference's parameter is more lenient (nullable) than the functional interface requirement, preventing false positives in valid code patterns.

• Tests

  • Added test coverage for method reference generic parameter nullability covariance scenarios.

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[msridhar]

Hi @utafrali at a high level this seems wrong to me. Can you point to where in the JLS / JSpecify specification this covariant mode for type arguments is discussed?

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 334016d and 4707366.

📒 Files selected for processing (3)

• nullaway/src/main/java/com/uber/nullaway/generics/CheckIdenticalNullabilityVisitor.java
• nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java
• nullaway/src/test/java/com/uber/nullaway/jspecify/GenericMethodLambdaOrMethodRefArgTests.java

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Walkthrough

This pull request adds covariant nullability checking for generic type parameters in method references. It modifies CheckIdenticalNullabilityVisitor to accept a covariant flag that alters mismatch detection: when enabled, only RHS @Nullable vs LHS non-null is considered a failure, not the reverse. A new helper method covariantTypeParameterNullability in GenericsChecks delegates to this covariant mode. Method reference parameter type relations now use covariant checking instead of the previous symmetric approach. Tests are added to verify the new behavior for method reference argument nullability.

Possibly related PRs

• Better handling of method references passed to generic methods in JSpecify mode #1499: Main PR extends method-reference generic-parameter nullability checks and uses GenericsUtils.processMethodRefTypeRelations functionality directly built upon in this PR.
• Account for annotations in extends / implements when computing view as supertype #1266: Both PRs modify CheckIdenticalNullabilityVisitor constructor, fields, and nullability-checking behavior.
• JSpecify: improve inference for generic methods based on method reference arguments #1438: Both PRs modify GenericsChecks and method-reference generic inference and parameter nullability handling.

Suggested labels

jspecify

Suggested reviewers

• yuxincs
• lazaroclapp

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 27.27% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: introducing covariant type handling for type parameter nullability matching in the visitor.
Linked Issues check	✅ Passed	Changes implement covariant nullability checking via a new mode in CheckIdenticalNullabilityVisitor and update GenericsChecks to use it for method reference parameters, directly addressing issue #1528's regression.
Out of Scope Changes check	✅ Passed	All changes are narrowly focused on fixing covariant type parameter nullability handling in method reference contexts, with no unrelated modifications detected.

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✨ Finishing Touches

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[utafrali]

Good point, let me double-check this against the spec and revise if needed.

[msridhar]

Closing as this fix is invalid; see #1528 (comment)