JSON.stringify answers undefined for an undefined, function, or symbol
root. Through `unknown` (or a generic instantiated with it) the dyn-root
lowering converted the runtime result to a string, so the result was the
text "undefined": `text === undefined` was false and `typeof text` was
"string".
Where the result reaches a use that distinguishes undefined from that text
(undefined/null comparisons, typeof, ??, ?., truth tests, a return from a
function declaring undefined, or an unannotated local binding used that way), the
call now keeps the checked `string | undefined` result the replacer path
already produces; later string uses of such a binding narrow by the
checker's flow type. Other uses keep the existing string ABI, so programs
that cannot observe the difference compile unchanged.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
