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Malte UblandClaude Opus 5.5
887aa774e0
`const message: M = { content: [] }` with `content: TextContent[] |
ImageContent[]` threw at run time, `a 'number[]' value is not
representable in the target union`. tsc types an empty literal `never[]`,
whose element has no representation of its own: the literal mapped through
never's f64 element as `number[]`, then the union slot's retag found no
width lift into either array arm and took the stranded-source trap.
An empty array carries no element to select an arm by, and it is the same
value whichever array family stores it, so the literal is built as the
union's FIRST array arm — the rule a lone array slot already states, and
the one the ternary arm and the `?? []` default apply. Both the IR slot
(lowerExprExpecting) and the literal's own type mapping (lowerArrayLiteral)
now state it for any number of array arms; a single array arm keeps the
identical lowering it had. A `never[]` VALUE (an explicitly declared
variable) still maps to `number[]` and keeps its refusal: mapping never's
element globally is a different change.
Corpus: tests/corpus/empty-array-union-slot.ts pins the nested record
field, declarations and reassignment, declared and async returns, ternary
arms (one and both empty), arguments and default parameters, deeper
nesting, a class field, an optional arm, array/Set/Map element slots, a
callback argument, an assertion, and a fresh array per evaluation.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>