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Add a case in our inference for generic method calls #1240

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msridhar merged 3 commits into from
Aug 8, 2025
Merged

Add a case in our inference for generic method calls #1240

msridhar merged 3 commits into from
Aug 8, 2025

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@msridhar msridhar commented Aug 7, 2025 •
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Fixes #1238

Before we were missing a fallback case; now we just use the type of the expression computed by javac (to be improved in the future).

Summary by CodeRabbit

  • Bug Fixes

    • Improved handling of nullable type variables in generic methods to ensure correct type inference in assignment contexts.

  • Tests

    • Added a new test to verify correct behavior when assigning non-nullable arrays to nullable array parameters in generic methods.

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coderabbitai bot commented Aug 7, 2025 •
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Walkthrough

A bug fix was made to the generic type inference logic for method calls in the NullAway static analysis tool, ensuring correct handling when the assignment context type is nullable and the type variable has a nullable upper bound. A new regression test was added to verify this fix for generic methods returning arrays.

Changes

Cohort / File(s) Change Summary
Generic type inference handling
nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java		 Modified inferGenericMethodCallType to explicitly substitute the type variable with the original expression type (exprType) when the assignment context is nullable and the type variable has a nullable upper bound.
Regression test for generic method inference
nullaway/src/test/java/com/uber/nullaway/jspecify/GenericMethodTests.java		 Added the issue1238 test to verify that a generic method with a nullable-bounded type variable correctly infers array types when assigned to a nullable array parameter.

Sequence Diagram(s)

sequenceDiagram
    participant Test as GenericMethodTests.issue1238()
    participant Mediator as CreatorMediator
    participant Bridge as WebFeedBridge

    Test->>Mediator: get(Key<byte[]>)
    Mediator-->>Test: byte[]
    Test->>Bridge: followFromId(byte[], ...)
    Bridge-->>Test: (no error)
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Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Assessment against linked issues

Objective Addressed Explanation
Fix crash when inferring generic return array type in assignment to nullable array parameter (#1238) ✅
Add regression test to reproduce and verify fix for generic method with nullable type variable returning arrays (#1238) ✅

Assessment against linked issues: Out-of-scope changes

No out-of-scope changes detected.

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📥 Commits

Reviewing files that changed from the base of the PR and between 92c9304 and a7a696a.

📒 Files selected for processing (2)
  • nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java (1 hunks)
  • nullaway/src/test/java/com/uber/nullaway/jspecify/GenericMethodTests.java (1 hunks)
🔇 Additional comments (2)
nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java (1)

517-521: LGTM! Excellent fix for the generic method call inference issue.

This fallback case correctly handles the scenario where both the assignment context type is nullable and the type variable has a nullable upper bound. By explicitly mapping the type variable to the original expression type (exprType), this prevents the ClassCastException described in issue #1238 where a TypeVar was incorrectly cast to an ArrayType. The fix ensures there's always a proper substitution in the map rather than leaving it undefined.

nullaway/src/test/java/com/uber/nullaway/jspecify/GenericMethodTests.java (1)

741-776: Excellent regression test for issue #1238.

This test comprehensively covers the bug scenario from the Chromium codebase:

  1. Generic method with nullable upper bound: <T extends @Nullable Object> T get(Key<T> key)
  2. Array type compatibility: byte[] (from Key<byte[]>) being passed to a parameter expecting byte @Nullable []
  3. Assignment context inference: The fix ensures proper type substitution when the generic method return type is a type variable with nullable upper bound

The test validates that the type inference correctly handles this case without crashing and allows the valid assignment. This directly validates the fix in GenericsChecks.java.

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  • Commit unit tests in branch issue-1238

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@msridhar msridhar requested a review from yuxincs August 7, 2025 22:41
@msridhar msridhar merged commit 7b64688 into master Aug 8, 2025
11 checks passed
@msridhar msridhar deleted the issue-1238 branch August 8, 2025 02:46
haewiful pushed a commit to haewiful/NullAway01 that referenced this pull request Aug 9, 2025
@msridhar @haewiful
Add a case in our inference for generic method calls (uber#1240)
39638f6 
Fixes uber#1238 

Before we were missing a fallback case; now we just use the type of the
expression computed by `javac` (to be improved in the future).

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved handling of nullable type variables in generic methods to
ensure correct type inference in assignment contexts.

* **Tests**
* Added a new test to verify correct behavior when assigning
non-nullable arrays to nullable array parameters in generic methods.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->
@coderabbitai coderabbitai bot mentioned this pull request Aug 15, 2025
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@msridhar msridhar mentioned this pull request Aug 18, 2025
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Crash wrt inferring generic return array type

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@msridhar @yuxincs