Emit error when trying to iterate a value with union type with non-iterable constituent when --downlevelIterator

[uhyo]

Fixes #39161 .

Example

code

function getArrayOrUndefined() {
  return Math.random() > 0.5 ? [] : undefined;
}

const result = [...getArrayOrUndefined()];

{
  "compilerOptions": {
    "target": "es5",
    "lib": ["es2015"],
    "strict": true,
    "downlevelIteration": true
  }
}

Current Behavior

(no compile error)

New Behavior

src/index.ts:5:20 - error TS2548: Type 'never[] | undefined' is not an array type or does not have a '[Symbol.iterator]()' method that returns an iterator.

5 const result = [...getArrayOrUndefined()];
                     ~~~~~~~~~~~~~~~~~~~~~


Found 1 error.

Details of Code Change

This PR adds a shouldIgnoreNonIterableUnionConstituent parameter to getIterationTypesOfIterable which, if false is specified, returns noIterationTypes whenever given union type includes a non-iterable constituent. Otherwise, non-iterable constituents are just ignored; for example, the yielded type of number[] | undefined is considered number.

I first considered making this the default behavior, but then one test failed (namely generatorYieldContextualType.ts). As the old behavior seemed kind of reasonable in some situations, I kept the change smaller by adding that parameter.

[jakebailey]

This seems fine to me, but it currently has a merge conflict; can you fix it?

[uhyo]

@jakebailey Thank you very much for noticing this PR!

However, the issue seems to have already been fixed by #40350.

#39161 and this PR can now be closed.

[jakebailey]

Wow, clearly I didn't note the date, sorry about that!

I tested this manually, and yeah, this is fixed.