Doug Gregor 4b3d2f47d9 [Typed throws] Handle function conversions involving different thrown errors ...

Teach the constraint solver about the subtyping rule that permits
converting one function type to another when the effective thrown error
type of one is a subtype of the effective thrown error type of the
other, using `any Error` for untyped throws and `Never` for
non-throwing.

With minor other fixes, this allows us to use typed throws for generic
functions that carry a typed error from their arguments through to
themselves, which is in effect a typed `rethrows`:

```swift
func mapArray<T, U, E: Error>(_ array: [T], body: (T) throws(E) -> U)
throws(E) -> [U] {
  var resultArray: [U] = .init()
  for value in array {
    resultArray.append(try body(value))
  }
  return resultArray
}
```

2023-10-05 11:55:05 -07:00

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