Use ground target types when handling lambdas and method refs passed to generic methods

[msridhar]

Fixes #1522

When handling lambdas and method references passed to a generic method, if the generic method's formal parameter type is a class type with wildcards at the top level, we need to compute the ground target type for checking. In the ground target type, wildcards are replaced with their lower bound if explicit, and otherwise replaced with their upper bound.

So, for the test from #1522:

@NullMarked
class Test {
  static class Foo<T> {
    public final <V> Foo<V> mapNotNull(Function<? super T, ? extends @Nullable V> mapper) {
      throw new RuntimeException();
    }
  }
  static <T> Foo<T> after(Foo<Optional<T>> foo) {
    return foo.mapNotNull(x -> x.orElse(null));
  }
}

We have a lambda passed to mapNotNull, whose parameter type is Function<? super T, ? extends @Nullable V>. The ground target type for this parameter is Function<T, @Nullable V>, which should be used to infer types at the call site and check the lambda. In this case, inference succeeds, and the return of x.orElse(null) from the lambda is fine since the return type is @Nullable V.

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Improved generic nullability inference for lambda expressions and method references by correctly grounding wildcard and parameterized types before applying nullability constraints.
  • Enhanced handling of complex nested wildcard type parameters in generic inference scenarios.

• Tests

  • Expanded test coverage for wildcard nullness behavior across lambdas, method references, and nested-wildcard type parameter scenarios.

[msridhar]

This change is part of the following stack:

• Run inference for generic method calls nested inside receivers #1571
  • Improve handling of var-declared local variables #1573
    • Compensate for more annotations inserted by javac #1574
      • Enable test for issue 1500 and add comment #1560
        • Use ground target types when handling lambdas and method refs passed to generic methods #1575 ◀
          • More generic inference for diamond calls #1544
          • Fix for unbounded wildcard passed to @NullUnmarked type variable #1577
            • Detect and warn on annotations directly on wildcard types #1579

_{Change managed by git-spice.}

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.20%. Comparing base (c315bd5) to head (38a6a9e).

Files with missing lines	Patch %	Lines
...java/com/uber/nullaway/generics/GenericsUtils.java	87.50%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1575      +/-   ##
============================================
- Coverage     88.30%   88.20%   -0.11%     
- Complexity     2926     2934       +8     
============================================
  Files           104      104              
  Lines          9776     9807      +31     
  Branches       1967     1974       +7     
============================================
+ Hits           8633     8650      +17     
- Misses          543      557      +14     
  Partials        600      600              

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[lazaroclapp]

Some questions just out of curiosity, but LGTM!

[lazaroclapp]

What does the true negative case look like here? Changing Function<Box<? super String>, R> mapper with Function<Box<? extends String>, R> mapper?

[msridhar]

Yes, that's correct. I've enhanced the test to show this.

[lazaroclapp]

This works the same regardless of whether we are looking at the types used for the arguments of the function vs the return argument of the function? This looks correct to me from 9.9, but feels a bit counter-intuitive...

[msridhar]

Yes, that's correct. It was counterintuitive to me too. It's basically strictly on the bounds of the wildcard type, independently of where it is used.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between c315bd5 and 38a6a9e.

📒 Files selected for processing (3)

• nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java
• nullaway/src/main/java/com/uber/nullaway/generics/GenericsUtils.java
• nullaway/src/test/java/com/uber/nullaway/jspecify/WildcardTests.java

---

Walkthrough

This PR implements target-type grounding for wildcard generics in NullAway's poly-expression (lambda and method-reference) nullability inference. A new groundTargetType utility in GenericsUtils replaces immediate wildcard type arguments with their bounds. The maybeStorePolyExpressionTypeFromTarget method is refactored to accept VisitorState and ground the target before restoring explicit nullability. Call sites throughout generic method inference and call-site checking apply grounding before storing inferred or explicit nullability for lambda and method-reference arguments. Four new tests replace a previously ignored test, validating nullness diagnostics for generic inference with null-returning expressions and nested wildcard preservation.

Possibly related PRs

• uber/NullAway#1553: Updates compareGenericTypeParameterNullabilityForCall to store method-reference poly expression types via maybeStorePolyExpressionTypeFromTarget, directly aligned with this PR's poly-expression caching in the same pipeline.
• uber/NullAway#1438: Changes GenericsChecks handling of method-reference generic nullability inference by extending inference to process method references instead of skipping them.
• uber/NullAway#1256: Instance-based GenericsChecks refactor that makes call sites use VisitorState directly, providing the foundation for this PR's state-aware maybeStorePolyExpressionTypeFromTarget.

Suggested labels

jspecify

Suggested reviewers

• lazaroclapp
• yuxincs

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 53.33% 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 clearly identifies the main change: using ground target types for lambdas and method references in generic methods.
Linked Issues check	✅ Passed	The PR implementation matches the objective from #1522: it fixes lambda/method reference handling for generic parameters with wildcards by introducing ground target types.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing issue #1522: implementing wildcard grounding in GenericsChecks, adding groundTargetType utility in GenericsUtils, and adding comprehensive test coverage.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-1522

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