Inference to `Iterable<[K, V]> | undefined` fails

[sandersn]

Regression in 4.1, found in adone on DT.

declare class DMap<K, V>  {
    constructor(iterable: Iterable<[K, V]> | undefined);
}
new DMap([["1", 2]])

Expected behavior:

K=string, V=number, no error

Actual behavior:
K=string | number, V=string | number, "(string | number)[][] is not assignable to to parameter of type Iterable<[string | number, string | number]>."

Removing undefined makes inference work; the actual workaround I used for adone was to split a constructor with optional parameters into 3 overloads.

[sandersn]

Also fails on 4.0 without --target esnext

[sandersn]

Bisecting shows that #40350 is at fault. This new error might be expected, but I'd still like to confirm.