[David Kaminski (dvkam)]

Fixes #65370

Change

Require the Iterator.prototype.flatMap mapper to return an iterator or iterable object. Primitive strings satisfy Iterable<string> in TypeScript, but ECMAScript rejects them when the result is consumed.

Tests

Added a compiler case that rejects a primitive string and preserves arrays, boxed strings, and iterator objects. Updated the affected baselines. The compiler baseline suite passed in default and concurrent modes on the reviewed patch.

I used AI assistance to prepare this change and personally reviewed the code and the issue report.

[Copilot]

🟢 Approval recommended

No active review issues were identified for the change requiring iterator flatMap mapper results to be objects.

No open issues 🎉

What changed in this PR

Fixes #65370 by aligning TypeScript’s Iterator.prototype.flatMap declaration with ECMAScript’s requirement that mapper results be objects, rejecting primitive strings.

Changes:

• Intersects the mapper’s return type with object.
• Adds regression coverage for rejected primitive strings and accepted arrays, iterator objects, and boxed strings.
• Updates type, symbol, and diagnostic baselines.

File	Description
tsc/​testdata/​tests/​cases/​compiler/​iteratorFlatMapRejectsPrimitiveString.ts	Tests rejected and supported mapper results.
tsc/​testdata/​baselines/​reference/​compiler/​iteratorFlatMapRejectsPrimitiveString.types	Records inferred types for the new test.
tsc/​testdata/​baselines/​reference/​compiler/​iteratorFlatMapRejectsPrimitiveString.symbols	Records symbol bindings for the new test.
tsc/​testdata/​baselines/​reference/​compiler/​iteratorFlatMapRejectsPrimitiveString.errors.txt	Captures the primitive-string rejection diagnostic.
tsc/​testdata/​baselines/​reference/​compiler/​builtinIterator.types	Updates existing snapshots of the flatMap signature.
tsc/​testdata/​baselines/​reference/​compiler/​builtinIterator.errors.txt	Updates existing diagnostics for the constrained return type.
tsc/​internal/​bundled/​libs/​lib.es2025.iterator.d.ts	Requires flatMap mapper results to be objects.

_{Review effort: Balanced}

[David Kaminski (dvkam)]

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