Substitute inferred @NonNull types for generic method inference

[msridhar]

Fixes #1444

In certain cases, like the test from #1444 added here, javac adds a @Nullable annotation to a type incorrectly as part of its inference. Here, for a call from the test:

@NullMarked
public class Test {
    static class DefaultConfiguration {
        final @Nullable Stream<? extends String> children;
    }
    final Stream<? extends String> children;
    Test(DefaultConfiguration configuration) {
        // relevant call
        this.children = notNull(configuration.children, "children must not be null");
    }
    public static <T> T notNull(@Nullable T object, String message) {
        throw new RuntimeException("todo");
    }
}

javacs inferred type for the return of the notNull call matches the inferred parameter type exactly, @Nullable Stream<...>. But, when applying JSpecify rules, the return cannot be @Nullable. To fix this, add inferred @NonNull qualifiers along with @Nullable when doing a substitution inside a type.

Summary by CodeRabbit

• Bug Fixes

  • Simplified printing of error/unknown generic types to show only the simple type name.
  • Improved type-variable substitution so inferred nullability is applied for both nullable and non-null cases.

• Tests

  • Added/updated tests covering generic method type inference and JSpecify/Nullability behaviors (includes a new scenario for issue1444).

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[msridhar]

Drive-by fix to make printed errors more readable, by printing e.g. @NonNull instead of @nullaway.synthetic.NonNull

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.40%. Comparing base (fc6d956) to head (9712145).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1445   +/-   ##
=========================================
  Coverage     88.40%   88.40%           
- Complexity     2711     2712    +1     
=========================================
  Files            99       99           
  Lines          9004     9007    +3     
  Branches       1799     1799           
=========================================
+ Hits           7960     7963    +3     
  Misses          517      517           
  Partials        527      527           

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[coderabbitai]

Walkthrough

Adds an override to GenericTypePrettyPrintingVisitor to print Type.ErrorType as the simple symbol name, and changes TypeSubstitutionUtils.substituteInferredNullabilityForTypeVariables to substitute inferred nullability for all matching type variables (emitting synthetic nullable or non-null annotations based on the inferred value). Tests were updated for expected diagnostic messages and two new tests (issue1444) were added covering contract/nonnull behavior.

Possibly related PRs

• PR 1348: Modifies substituteInferredNullabilityForTypeVariables in TypeSubstitutionUtils with related inferred-nullability propagation changes.
• PR 1371: Updates nullability annotation restoration/substitution for generics when computing method return/field types.
• PR 1248: Refactors generic type-argument substitution and inferred-nullability application used when substituting type args in generic method types.

Suggested reviewers

• yuxincs
• lazaroclapp

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 12.50% 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 accurately describes the main change: adding support for substituting inferred @NonNull types during generic method type inference to fix nullable return type inference issues.
Linked Issues check	✅ Passed	The changes directly address issue #1444 by adding @NonNull substitutions in generic inference, handling error types, and adding a test case that reproduces the reported failure scenario.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing generic method type inference for nullable return types as described in the linked issue; no out-of-scope modifications detected.

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✨ Finishing touches

• 📝 Generate docstrings

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