Destructuring assignment makes any types when it skips some elements (regression)

[falsandtru]

cc @andy-ms

TypeScript Version: 2.7.0-dev.20171223

Code

const [, a = ''] = ''.match('') || [];

Expected behavior:

a is string.

Actual behavior:

a is any.

[weswigham]

@ahejlsberg The bisect indicates that 25af351 introduced this regression as part of #19671. I think the change is unintentional, in this instance? Before, when we saw RegExpMatchArray on the LHS of || and [any, ""] on the RHS, we did a best-choice thing that eliminated the tuple type from the union (neither is a subtype of the other thanks to tuples and any, I think, but the tuple is assignable to the regexp match array, so was eliminated before). Now that it is preserved, when we go to get the index type for the union, we see a tuple with an any member (created from the shape of the destructuring pattern) and happily state that the numeric index type must thusly be any. I think a decent fix that doesn't revert our behavior back to the best-choice mechanic is to stop using unknownType for positions indicated by OmittedExpressions when building the tuple type in the first place in checkArrayLiteral (since those broaden the index type of the tuple type created to any), and instead use emptyObjectType.

[falsandtru]

Also see #20495