[briankeefe]

Problem

util.callbackify() applies the intrinsic Promise.prototype.then to its return value, so ordinary thenables throw instead of delivering their result. This reproduces the thenable failure in parallel/test-util-callbackify.js in the current Node-compat results.

Change

Invoke the returned object's own then through FunctionPrototypeCall, preserving its receiver and the existing next-tick delivery and falsy-rejection handling. Add fulfillment/rejection regressions. No vendored fixtures, skips, or compatibility expectations change.

Verification

• Built the edited Deno source with ./x build on Linux x86_64.
• The two added regression bodies, copied unchanged into a standalone runner with the unit file's process initialization: released Deno 2.9.8 0 passed / 2 failed; source-built patched Deno 2 passed / 0 failed.
• Additional real-runtime checks passed for native Promises, thenables, receivers, argument forwarding, next-tick delivery, falsy rejections, return value, and synchronous exceptions. Node v26.12.1 provides the behavior oracle.
• Project JS lint, project formatter checks on both changed files, and unit-file type checking passed.

The Cargo test harnesses and full CI have not been run. The unchanged upstream compatibility fixture advances past the thenable cases but still fails a separate falsy-rejection stack-frame assertion, also reproducible on released Deno. It is not marked passing or skipped by this patch.

AI disclosure: The implementation and tests were authored using OMP with GPT-6.1 Sol and received a separate AI-assisted review. The runtime/test results above were actually executed; this draft does not claim human code review or full-suite success.

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