Better handling of method references passed to generic methods in JSpecify mode by msridhar · Pull Request #1499 · uber/NullAway

msridhar · 2026-03-25T02:38:24Z

We make two key changes. First, we store the inferred types of method reference expressions in the inferredPolyExpressionTypes cache, similar to what we did for lambdas, so that some later checks that require the type of the method reference get the inferred type.

Second, when a method reference is passed to a generic method, sometimes the type for the method reference computed by javac does not include nullability annotations on nested types (see #1474 and a new test case here). So, we need to do explicit checks using the return and parameter types of the referenced method, to see if nested nullability annotations are consistent with the target functional-interface method type.

For code reuse and clarity, we extract a utility method for doing type checks related to method references to a new class GenericsUtils, and rewrite one use of the original logic to use the new utility.

Summary by CodeRabbit

New Features
- Added nullability checks for method references when used with generic type inference.
Improvements
- Brought method-reference type handling in line with lambda expressions, improving detection of parameter and return nullability mismatches, including varargs and explicit type-argument scenarios.
Tests
- Added tests covering method-reference interactions with generic inference and expected nullability diagnostics.

codecov · 2026-03-25T02:44:56Z

Codecov Report

❌ Patch coverage is 88.59649% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.45%. Comparing base (f4dcfc9) to head (9c5c926).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...java/com/uber/nullaway/generics/GenericsUtils.java	85.93%	3 Missing and 6 partials ⚠️
...ava/com/uber/nullaway/generics/GenericsChecks.java	92.00%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1499      +/-   ##
============================================
+ Coverage     88.43%   88.45%   +0.01%     
- Complexity     2813     2823      +10     
============================================
  Files            99      100       +1     
  Lines          9410     9446      +36     
  Branches       1888     1893       +5     
============================================
+ Hits           8322     8355      +33     
- Misses          529      530       +1     
- Partials        559      561       +2

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coderabbitai · 2026-03-26T22:14:19Z

No actionable comments were generated in the recent review. 🎉

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nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java
nullaway/src/main/java/com/uber/nullaway/generics/GenericsUtils.java

Walkthrough

Adds method-reference support to generic-method inference and assignment-compatibility checks by treating MemberReferenceTree as a poly-expression, generalizing target-type caching, and delegating method-reference return/parameter relation generation to a new utility class GenericsUtils. Exposes two GenericsChecks helpers as package-visible, introduces method-reference-specific error reporters, substitutes explicit type arguments for referenced methods, and adds/updates tests covering method-reference inference, nested nullability, and array/varargs cases.

Possibly related PRs

JSpecify: improve inference for generic methods based on method reference arguments #1438: Implements method-reference-related changes to generics inference and updates GenericsChecks for MemberReferenceTree handling.
JSpecify: Apply annotations on type variables to lambdas in more cases #1428: Generalizes lambda target-type caching/inference; this PR extends that caching to poly-expressions including method references.
Better handle calls to super constructors and superclass methods in JSpecify mode #1248: Modifies the same GenericsChecks functions (type resolution and generic-parameter nullability logic) touched by this change.

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 37.50% 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 'Better handling of method references passed to generic methods in JSpecify mode' directly describes the main change of integrating method-reference types with generic method type inference.
Linked Issues check	✅ Passed	All coding requirements from `#1431` and `#1474` are met: method-reference types are cached for type inference, explicit return/parameter nullability checks detect nested annotation mismatches, and varargs array contents nullability is properly handled.
Out of Scope Changes check	✅ Passed	All code changes are within scope: modifications to GenericsChecks handle method references, new GenericsUtils class encapsulates method-reference logic, and tests validate the fixes for issues `#1431` and `#1474`.

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Commit unit tests in branch codex/add-test-and-fix-nullaway-issue-1431

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coderabbitai

Actionable comments posted: 1

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Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@nullaway/src/main/java/com/uber/nullaway/generics/GenericsUtils.java`:
- Around line 66-69: The code currently force-casts the result of
genericsChecks.getMemberReferenceMethodType(...) via castToNonNull into
referencedMethodType, which can be null outside JSpecify mode; update the logic
in GenericsUtils to first call
genericsChecks.getMemberReferenceMethodType(memberReferenceTree,
referencedMethod, state), check for null and return early (or handle the null
case appropriately) before calling castToNonNull, so castToNonNull is only
invoked on a guaranteed non-null value.

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Reviewing files that changed from the base of the PR and between e78dc89 and 63ab186.

📒 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/GenericMethodLambdaOrMethodRefArgTests.java

msridhar · 2026-03-26T22:15:29Z

+    if (typeArgumentTrees != null
+        && !typeArgumentTrees.isEmpty()
+        && overridingMethod.asType() instanceof Type.ForAll forAllType) {
+      // handle explicit type arguments in method reference, e.g., x::<Foo>method


The other changes in this PR exposed this issue; apparently we weren't doing the appropriate substitution before when a method reference has explicit type arguments

Is there a case where a method reference has some type arguments given explicitly and some inferred? (Not sure if this is possible in Java, just checking)

No, it either needs to be all explicit or all inferred

lazaroclapp

Did my best to check this. LGMT. Biggest thing bothering me is the location of the annotation in the error messages, but I think that's just our existing pretty-printing logic instead?

lazaroclapp · 2026-03-29T18:17:00Z

+      return;
+    }
+    Type.MethodType referencedMethodType =
+        castToNonNull(


Why is this cast safe? And why is it preferred to an if check and return? I suspect it might be both, just noting that it might benefit from a comment as to why :)

Edit: There was a null check and an alternative default for referencedMethodType in the non-refactored version, no?

You're right. A null check does no harm here (though I don't think we should ever see null). Will add.

lazaroclapp · 2026-03-29T18:25:00Z


  private final NullAway analysis;
-  private final Config config;
+  /* package-private */ final Config config;


Why? Can't get pass it around or create a package-private accessor?

Good catch, added a package-private getter

lazaroclapp · 2026-03-29T18:26:51Z

-      solver.addSubtypeConstraint(fiParamTypes.get(fiStartIndex + i), varargsElementType, false);
-    }
+        state,
+        (subtype, supertype, relationKind) -> {


I guess relationKind is used only for reporting and then only when checking call-sites, right?

Does java support something like: (subtype, supertype, _) -> { here for clarity?

Java does support _ but only in a more recent version than we are using. I renamed to unused here; you're right this is only used in the other case for error reporting.

lazaroclapp · 2026-03-29T18:31:15Z

+    if (typeArgumentTrees != null
+        && !typeArgumentTrees.isEmpty()
+        && overridingMethod.asType() instanceof Type.ForAll forAllType) {
+      // handle explicit type arguments in method reference, e.g., x::<Foo>method


Is there a case where a method reference has some type arguments given explicitly and some inferred? (Not sure if this is possible in Java, just checking)

@nonnull

See #1499 (comment). This should make our error messages more consistent and less confusing. We now only include `@Nullable` type use annotations. Eventually, we may want to adopt [JSpecify shorthand](https://github.com/jspecify/jspecify/wiki/notation#shorthand-notation) when printing types, but we'd have to think more carefully about that.  ## Summary by CodeRabbit * **Refactor** * Diagnostic type annotations are printed more concisely: most inline annotations (e.g., @nonnull) are suppressed while @nullable remains visible once; array spacing normalized for clearer output. * **Tests** * Updated expected diagnostic texts in several tests to match the slimmer annotation formatting.

…ks.java Co-authored-by: Lázaro Clapp <lazaro@uber.com>

Integrate method reference target types into generic inference checks

62acd0a

msridhar added the codex label Mar 25, 2026 — with ChatGPT Codex Connector

msridhar added 13 commits March 25, 2026 09:50

Merge branch 'master' into codex/add-test-and-fix-nullaway-issue-1431

ae76bce

another test

777945d

more fixes

a6d7b4c

more

ee83c45

better error message

401bcc5

refactoring WIP

e9310cf

bug fix

4989c67

tweak

70b55f1

tweaks

1f75f7b

various comments and tweaks

fb87e20

more

af7f546

rename

b2dd756

a bit more

63ab186

msridhar marked this pull request as ready for review March 26, 2026 22:09

msridhar changed the title ~~Handle method references in poly-expression nullability inference and add tests~~ Better handling of method references passed to generic methods in JSpecify mode Mar 26, 2026

coderabbitai Bot reviewed Mar 26, 2026

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Comment thread nullaway/src/main/java/com/uber/nullaway/generics/GenericsUtils.java Resolved

msridhar commented Mar 26, 2026

View reviewed changes

msridhar requested a review from lazaroclapp March 26, 2026 22:17

msridhar mentioned this pull request Mar 26, 2026

Address nullness warning from upcoming version of NullAway spring-projects/spring-framework#36548

Closed

lazaroclapp approved these changes Mar 29, 2026

View reviewed changes

Merge branch 'master' into codex/add-test-and-fix-nullaway-issue-1431

1d55d77

msridhar mentioned this pull request Apr 1, 2026

Print only @Nullable type use annotations in error messages #1507

Merged

msridhar added 3 commits April 3, 2026 08:58

Merge branch 'master' into codex/add-test-and-fix-nullaway-issue-1431

23f84c3

fix expected messages

b2ada24

add null check

8800977

msridhar and others added 3 commits April 3, 2026 12:59

add getter

96f0b68

rename variable

38070d0

Update nullaway/src/main/java/com/uber/nullaway/generics/GenericsChec…

9c5c926

…ks.java Co-authored-by: Lázaro Clapp <lazaro@uber.com>

msridhar enabled auto-merge (squash) April 3, 2026 20:09

msridhar merged commit cf60f58 into master Apr 3, 2026
11 of 12 checks passed

msridhar deleted the codex/add-test-and-fix-nullaway-issue-1431 branch April 3, 2026 20:11

coderabbitai Bot mentioned this pull request Apr 4, 2026

Handle nested restrictive annotations on type variables in @NullUnmarked code #1510

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3 tasks done

coderabbitai Bot mentioned this pull request Apr 14, 2026

Fix type parameter nullability matching for covariant types #1533

Closed

This was referenced Jun 10, 2026

Add support for filtering null objects from streams #1597

Merged

Better support for conditional expressions in JSpecify mode #1608

Merged

coderabbitai Bot mentioned this pull request Jun 24, 2026

Fix issue with passing poly expression to a function parameter with type from library model #1614

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coderabbitai Bot mentioned this pull request Jul 24, 2026

Fix bug with lambdas assigned to locals with a wildcard in their type #1647

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coderabbitai Bot mentioned this pull request Aug 11, 2026

Pretty-print substituted types in bad-override error messages #1697

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