[danlapid]

TestFixture::SetupParams had no initializers on ignoreAllAutogatesEnv, useRealTimers and flushActorStorage. Aggregate initialization ({...}) zeroes them, so the existing callers were fine. The new "reports an internal exception to the isolate's observer and the request's, once" case in uncaught-exception-test.c++ declares TestFixture::SetupParams params;, which leaves them indeterminate. buildConfig() then reads ignoreAllAutogatesEnv, and UBSan aborts:

src/workerd/tests/test-fixture.c++:287:57: runtime error: load of value 226, which is not a valid value for type 'bool'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior src/workerd/tests/test-fixture.c++:287:57

This fails @workerd//src/workerd/io:uncaught-exception-test (both variants) in the ubsan job of every edgeworker master pipeline since the test landed: 16 of 16 that reached the job on 2026-11-12.

Change

• Give the three bools = false.
• Clang rejects member initializers that the enclosing class's own in-class default argument depends on ("default member initializer ... needed within definition of enclosing class"). So TestFixture(SetupParams&& params = {.useRealTimers = false}) becomes a parameterless constructor that delegates, plus TestFixture(SetupParams&&).

Testing

• bazel test --config=ubsan --runs_per_test=5 @workerd//src/workerd/io:uncaught-exception-test@ @workerd//src/workerd/io:uncaught-exception-test@all-autogates: 5/5 pass. Without the change, the same command fails with the error above.
• Every test target that depends on //src/workerd/tests:test-fixture builds. bench-pumpto and bench-stream-piping fail at this base too: they still use the removed jsg::BackingStore.

🤖 Generated with Claude Code

[clanky-the-clanker]

✅ Approved — This is a small, correct fix for reading uninitialized bools in a test fixture. The constructor change is needed for the new initializers and keeps every existing call site working.

_{The decision is advisory: this review is a comment and never approves, blocks or requests changes on the pull request.}

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✅ All checks passed

📋 Code Quality: ✅ LGTM

The default member initializers fix the indeterminate bools in a default-initialized SetupParams. The delegating parameterless constructor keeps the old default-argument behaviour and avoids Clang's error about an incomplete enclosing class.

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📝 Documentation: ✅ LGTM

The new comment above the constructors accurately describes why they are written this way.

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_{Reviewed commit: ab282f1a}