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Chris Tate
dd3ea7c00c
`.filter` with a written type predicate (`(p): p is Cat => ...`, or a guard function passed as a value such as `filter(isDog)`) was refused with SC1090, because TypeScript does not check the predicate. An inferred predicate that keeps several union arms (`v => v !== undefined` over `(string | number | undefined)[]`) was refused too. The filter loop now keeps the receiver's element type, and the result converts to the predicate's type element by element (filter-predicates.ts): - a union element to one of its arms, or to a union of some of its arms, by its tag; - a checked-dynamic element by dynCheck; - a class instance to a subclass by its class; - a tuple or record to one with the same fields, field by field. An element the predicate's type does not describe, kept because the predicate returned true for it, throws a catchable TypeError: "a kept element is not a 'Cat' (a type predicate returned true for a value of another type)". The diagnostics fixture filter-narrow.ts, which pinned the three refusals, compiles now; corpus 7007 covers its cases. Fixes #1680.