Fix crashes when typing implicitly typed lambda parameters during inference by smillst · Pull Request #8023 · typetools/checker-framework

smillst · 2026-08-20T22:49:01Z

Reverts #7969 and fixes the underlying problem it worked around. Fixes #7681.

Background

#7969 avoided a set of crashes by refusing to compute a lambda's functional type whenever any
type argument inference was in progress, falling back to javac's Java type plus default
qualifiers. That is unsound rather than merely imprecise -- it strengthens a @Nullable String
parameter to @NonNull String -- and it fires even when the running inference could answer
correctly.

Root cause

Two ordering problems, both around the type of an implicitly typed lambda parameter.

1. A lambda body was typed before its parameters had types.
createAdditionalArgConstraints descended into a lambda argument's body while building the
constraint set, and computing the type of an invocation there requires the lambda's parameter
types. JLS 18.2.1 says a constraint on an implicitly typed lambda whose function type has a
non-proper parameter type reduces to false, and notes the condition "never arises in practice"
precisely because JLS 18.5.2.2 resolves the constraint's input variables first. The CF did not
follow that ordering.

2. A parameter's type was obtained by re-deriving the lambda's target type, which re-runs
inference for the invocation the lambda is an argument of. That makes one inference problem
depend on another, and the dependency can be a cycle:

run(promise -> ...)              inference for run(...) starts
  myStream(results)              started while run(...) creates constraints
    needs the type of `results`
    re-derives the lambda's target type
    promise.then(...)            started by that request
      needs the type of myStream(results), which is already running

The inner inference then answers with un-substituted types, and a type variable escapes as the
parameter's type, crashing later in AnnotatedTypes.asMemberOf -- or immediately in
assertIsFunctionalInterface when the target type is itself an inference variable.

The fix

Enforce the JLS 18.5.2.2 ordering. New LambdaBodyConstraint stands in for the constraints
produced by a lambda body. Its input variables are the inference variables mentioned by the
function type's parameter types, so getB4 resolves them before reducing it; the body's
constraints are created when it reduces. Constraint.Kind.LAMBDA_BODY is added, and
ConstraintSet.getClosedSubset treats it like EXPRESSION so the input variables are respected.

Answer from the inference that determined the type, rather than re-deriving.
Java8InferenceContext records each implicitly typed lambda parameter's target type and index
when the lambda's constraints are created; InvocationTypeInference.getLambdaParameterType
answers from that record. No re-derivation, so no cycle. When the record cannot supply a proper
type, the previous behavior runs unchanged -- in particular there is no fallback to javac's type
plus default qualifiers.

Keep the record after its inference ends. The record lived only as long as its inference was
on java8InferenceStack, so requests arriving afterwards -- including the first one of all, from
BaseTypeVisitor with an empty stack -- still re-derived. Resolved types are now recorded on
AnnotatedTypeFactory after b4.resolve(), cleared per compilation unit.

Two guards on what may be recorded or returned:

Reject a type mentioning a type variable declared by a method or constructor.
AbstractType.isProper() means only "mentions no inference variable of this problem", so a
method type variable belonging to a different, unfinished inference passes it. In Issue7698
that let myStream's T escape as a parameter's type.
Record only a type that agrees with javac's type for the parameter. Two inference problems can
record the same parameter, and the enclosing one is not the more authoritative:
createAdditionalArgConstraintsForInvocation rebuilds a nested invocation's constraints with
createC alone, omitting the applicability constraints that pin its type arguments. In
Issue1983.java that leaves F unconstrained, so the enclosing inference sees
Function<? super Object, ...> where javac has Function<? super Object[], ...>.

This PR replaces #8008 and #8009. This PR includes all the tests add in those two PRs.

…erence is currently running (typetools#7969)" This reverts commit a3775ba.

…ers have types JLS 18.2.1 says that a constraint on an implicitly typed lambda whose function type has a parameter type that is not a proper type reduces to false, and notes that the condition never arises in practice, because JLS 18.5.2.2 resolves the constraint's input variables first. createAdditionalArgConstraints did not follow that ordering: it descended into a lambda argument's body while creating constraints, and computing the type of an invocation there requires the lambda's parameter types. When those were not known yet, re-deriving the lambda's target type re-entered the enclosing invocation's inference, which deliberately answers with un-substituted types. The resulting inference variable then escaped as the parameter's type and crashed later in AnnotatedTypes.asMemberOf, or immediately in assertIsFunctionalInterface when the target type was itself an inference variable. Add LambdaBodyConstraint, whose input variables are the ones that the function type's parameter types mention, so that InvocationTypeInference.getB4 resolves them before reducing it, and create the body's constraints when it is reduced. Fixes the crashes in framework/tests/all-systems/Issue7677.java, checker/tests/all-systems/LambdaParams.java, and checker/tests/all-systems/java8inference/DeferredAdditionalArg.java. framework/tests/all-systems/Issue7698.java still crashes, for a different reason: the improper type there comes from another invocation whose own inference has not yet built a bound set. Also adds checker/tests/nullness/LambdaParamTypeNotCached.java, a test that a nested lambda's parameter keeps the type that inference gives it. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

The type of an implicitly typed lambda parameter was obtained by re-deriving the lambda's target type, which re-runs inference for the invocation that the lambda is an argument of. That makes one inference problem depend on another, and the dependency can be a cycle: run(promise -> ...) inference for run(...) starts myStream(results) started while run(...) creates constraints needs the type of `results` re-derives the lambda's target type promise.then(...) started by that request needs the type of myStream(results), which is already running Inference for myStream(results) is then asked for its type before its type arguments have been substituted, and answers with MyStream<T>. The type variable escapes as the type of `result`, and the crash surfaces later, in AnnotatedTypes.asMemberOf. The inference that gives a lambda its target type already knows where the lambda's parameters get their types from, so record that when the lambda's constraints are created, and answer requests from the record. Then no re-derivation happens and no cycle forms. When the record cannot supply a proper type, the previous behavior is used unchanged; in particular there is no fallback to javac's type plus default qualifiers, which would be unsound. Fixes the crash in framework/tests/all-systems/Issue7698.java. With the preceding commit, all four of Issue7677.java, Issue7698.java, checker/tests/all-systems/LambdaParams.java, and checker/tests/all-systems/java8inference/DeferredAdditionalArg.java type-check, and `./gradlew :checker:allNullnessTests` passes. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

… ends Fixes the remaining Issue7698 crash. Three parts: AbstractType.isProper() means only "mentions no inference variable of this problem". A lambda's target type can mention a method type variable of a different, unfinished inference -- in Issue7698, `map`'s function type is `Function<? super T, ? extends R>` where `T` belongs to the still-running inference for the receiver `myStream(results)`. That `T` is Kind.PROPER, so getLambdaParameterType returned it, and it escaped as the parameter's type and later failed in AnnotatedTypes.asMemberOf. Add a second guard rejecting a type that mentions a type variable declared by a method or constructor. The parameter-type record lived in Java8InferenceContext, so it was reachable only while its own inference was on java8InferenceStack. Requests arriving after that inference popped -- including the first one of all, from BaseTypeVisitor with an empty stack -- fell back to re-deriving the lambda's target type, which is what forms the cycle between two inference problems. Record the resolved types on AnnotatedTypeFactory after b4.resolve(), cleared per compilation unit, and consult them before re-deriving. Record a type only when it agrees with javac's type for the parameter. Two inference problems can record the same parameter, and the outer one is not the more authoritative: createAdditionalArgConstraintsForInvocation rebuilds a nested invocation's constraints with createC alone, omitting the applicability constraints that pin its type arguments. In Issue1983.java that leaves `F` unconstrained, so the inference for func1(transform(params, p -> ...)) resolves the lambda's target type to `Function<? super Object, ...>` where javac has `Function<? super Object[], ...>`, and recording it replaced a correct record with one that made `p[0]` fail to type. Verified with -AconvertTypeArgInferenceCrashToWarning=false, which the JUnit harness passes and which the previous measurements omitted: Issue7677, Issue7698, LambdaParams, DeferredAdditionalArg, and Issue1983 all type-check without a crash. :checker:allNullnessTests passes. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

@PARAM

The revert of typetools#7969 removed the @PARAM tags for `f` and `paramElement` along with the code that used them, and `lambdaParam` was added later without one. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

coderabbitai · 2026-08-20T22:52:10Z

📝 Walkthrough

Walkthrough

The framework now tracks inferred types for implicitly typed lambda parameters during Java 8 type inference. It defers lambda-body constraints until parameter types become proper, then resolves and records compatible types. Annotated type computation uses active or recorded types and clears recorded data when the compilation root changes. New nullness, tainting, and framework regression tests cover nested lambdas, generic inference, and nullable or tainted parameters.

Suggested reviewers: mernst

Merge Risk: 🟡 Moderate · up to 9c1bd

The PR changes lambda inference and can still leave stale parameter types cached or allow partially attributed types to trigger inference crashes, while one regression test does not match the implemented lifecycle contract. Merge should wait until these bounded correctness risks are addressed or explicitly accepted.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 44.29% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 70 functions across 17 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The implementation defers lambda-body constraints and records inferred parameter types, with Issue7681 covering the reported nested generic inference crash.
Out of Scope Changes check	✅ Passed	The production changes and regression tests focus on implicitly typed lambda inference and the linked crash scenario.

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Inline comments:
In
`@framework/src/main/java/org/checkerframework/framework/util/typeinference8/InvocationTypeInference.java`:
- Around line 204-220: Update mentionsMethodTypeVariable to also recursively
inspect DeclaredType.getEnclosingType() alongside the declared type’s own type
arguments, ensuring enclosing arguments such as Outer<T> in Outer<T>.Inner are
detected. Add a regression case covering an implicitly typed lambda parameter
using this nested type with an unfinished method type variable.

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📒 Files selected for processing (16)

checker/tests/nullness/LambdaBodyNestedInvocation.java
checker/tests/nullness/LambdaParamTypeFromInference.java
checker/tests/nullness/LambdaParamTypeNestedLambda.java
checker/tests/nullness/LambdaParamTypeNotCached.java
checker/tests/tainting/LambdaParamTypeFromInference.java
framework/src/main/java/org/checkerframework/framework/type/AnnotatedTypeFactory.java
framework/src/main/java/org/checkerframework/framework/type/TypeFromMemberVisitor.java
framework/src/main/java/org/checkerframework/framework/util/typeinference8/DefaultTypeArgumentInference.java
framework/src/main/java/org/checkerframework/framework/util/typeinference8/InvocationTypeInference.java
framework/src/main/java/org/checkerframework/framework/util/typeinference8/TypeArgumentInference.java
framework/src/main/java/org/checkerframework/framework/util/typeinference8/constraint/Constraint.java
framework/src/main/java/org/checkerframework/framework/util/typeinference8/constraint/ConstraintSet.java
framework/src/main/java/org/checkerframework/framework/util/typeinference8/constraint/LambdaBodyConstraint.java
framework/src/main/java/org/checkerframework/framework/util/typeinference8/util/Java8InferenceContext.java
framework/tests/all-systems/Issue7681.java
framework/tests/all-systems/NestedLambdaParameter.java

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Merge LambdaBodyNestedInvocation, LambdaParamTypeNestedLambda, and LambdaParamTypeNotCached into LambdaParamTypeFromInference, each as a static nested class. Nesting keeps the differing `map` overloads in separate scopes, so the calls in each test resolve as they did before. LambdaParamTypeNotCached duplicated LambdaParamTypeNestedLambda's helpers and dereference test, so those two became a single nested class NestedLambda; the block-bodied variant is kept as dereferenceBlockBody. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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Inline comments:
In `@checker/tests/nullness/LambdaParamTypeFromInference.java`:
- Around line 25-27: Update the comment describing lambda parameter type
resolution to state that the parameter target type is recorded during the
original map inference and retrieved after inference completes, rather than
recomputing inference or querying the running inference again.

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checker/tests/nullness/LambdaParamTypeFromInference.java

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mentionsMethodTypeVariable examined only a declared type's own type arguments, so it missed a type variable that appears solely in the enclosing type: in `Outer<T>.Inner`, `Inner` has no type arguments and `T` is mentioned only by `Outer<T>`. getLambdaParameterType would then supply such a type as an implicitly typed lambda parameter's type, which lets the type variable escape. The new test reaches the enclosing-type case; no existing test in framework/tests/all-systems or checker/tests/nullness does. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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Inline comments:
In
`@framework/src/main/java/org/checkerframework/framework/util/typeinference8/InvocationTypeInference.java`:
- Around line 208-215: Update mentionsMethodTypeVariable for TypeKind.ERROR to
conservatively return true, or recursively inspect its available type arguments
and enclosing type before returning false. Ensure getLambdaParameterType cannot
allow method type variables through unresolved error types, and add a regression
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mernst

Thanks! I left a few comments about documentation.

…eter-2

…r-framework into altern-lambda-parameter-2

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framework/src/main/java/org/checkerframework/framework/type/AnnotatedTypeFactory.java (1)
1692-1698: 🎯 Functional Correctness | 🟠 Major | ⚡ Quick win

Invalidate fromMemberTreeCache for the lambda parameter.

recordLambdaParameterType removes elementCache, but fromMember reads fromMemberTreeCache before it invokes TypeFromMemberVisitor. If code queried the VariableTree before inference completed, that cache retains the provisional type after this method records the final type. Remove the parameter declaration tree from fromMemberTreeCache in this invalidation block.
Proposed fix
   if (shouldCache) {
     // A request made before inference finished may have cached a type computed from a target
     // type that was not yet known.
     elementCache.remove(param);
+    Tree declaration = declarationFromElement(param);
+    if (declaration != null) {
+      fromMemberTreeCache.remove(declaration);
+    }
   }
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In
`@framework/src/main/java/org/checkerframework/framework/type/AnnotatedTypeFactory.java`
around lines 1692 - 1698, Update recordLambdaParameterType to also invalidate
fromMemberTreeCache for the lambda parameter’s declaration tree when shouldCache
is true, alongside the existing elementCache removal, so subsequent fromMember
queries recompute the finalized type.

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Outside diff comments:
In
`@framework/src/main/java/org/checkerframework/framework/type/AnnotatedTypeFactory.java`:
- Around line 1692-1698: Update recordLambdaParameterType to also invalidate
fromMemberTreeCache for the lambda parameter’s declaration tree when shouldCache
is true, alongside the existing elementCache removal, so subsequent fromMember
queries recompute the finalized type.

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framework/src/main/java/org/checkerframework/framework/util/typeinference8/InvocationTypeInference.java
framework/src/main/java/org/checkerframework/framework/util/typeinference8/TypeArgumentInference.java

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smillst and others added 7 commits August 20, 2026 15:28

Revert "Don't try to compute the functional type from a lambda if inf…

c8f9d88

…erence is currently running (typetools#7969)" This reverts commit a3775ba.

Add test cases.

4d96bdc

Add more tests.

876923a

Document the parameters of inferLambdaParamAnnotations

1aa6bcd

The revert of typetools#7969 removed the @PARAM tags for `f` and `paramElement` along with the code that used them, and `lambdaParam` was added later without one. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

This was referenced Aug 20, 2026

Get the type of lambda parameters from inference if it is running #8009

Closed

Type inference: incorporate the bound set of a nested poly invocation #8008

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Comment thread ...rc/main/java/org/checkerframework/framework/util/typeinference8/InvocationTypeInference.java

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Comment thread ...rc/main/java/org/checkerframework/framework/util/typeinference8/InvocationTypeInference.java

Merge ../checker-framework-branch-master into altern-lambda-parameter-2

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mernst added 2 commits August 21, 2026 08:34

Typo fix

a79d663

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4e7e3a3

mernst dismissed their stale review via 4e7e3a3 August 21, 2026 15:35

smillst added 3 commits August 21, 2026 10:01

Merge remote-tracking branch 'origin/master' into altern-lambda-param…

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…eter-2

Tweak comments.

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Merge branch 'altern-lambda-parameter-2' of github.com:smillst/checke…

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…r-framework into altern-lambda-parameter-2

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Merge ../checker-framework-branch-master into altern-lambda-parameter-2

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Fix crashes when typing implicitly typed lambda parameters during inference - #8023

Fix crashes when typing implicitly typed lambda parameters during inference#8023
smillst merged 16 commits into
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smillst commented Aug 20, 2026

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