new Map() infers type only from the first item in the iterable

TypeScript Version: 3.8.3

Search Terms: map infer type first item

Code

type Lookup = Map<number, boolean | string>;

const foo = new Map([
    [1, true],
    [2, 'foo']
]);

const bar: Lookup = new Map([
    [1, true],
    [2, 'foo']
]);

const baz: Lookup = new Map<number, boolean | string>([
    [1, true],
    [2, 'foo']
]);

Expected behavior: All 3 assignments work.

Actual behavior: The first 2 assignments fail because the type of the map is inferred as Map<number, () => boolean> (because of the first item). The 3rd one works but duplicates the type of the map.

Playground Link

Start by reproducing the TypeScript 3.8.3 behavior in the linked Playground using the three Map examples. Trace how the checker infers the type of the iterable passed to the Map constructor, then verify that all three assignments accept mixed boolean and string values without requiring a duplicated explicit type.