Narrowing strangeness with instanceof

[yortus]

TypeScript Version: nightly (2.1.0-dev.20160804)

Code

function f(s: Set<string> | Set<number>) {

    // (1)
    s = new Set<number>();
    s // s is Set<number>
    if (s instanceof Set) { } // Clearly no side-effects here. No possible changes to s.
    s // s is Set<number> | Set<any>                                                 <===== (1a)
    s.add(42); // ERROR: Cannot invoke an expression whose type lacks call signature <===== (1b)

    // (2)
    s = new Set<number>();
    s // s is Set<number>
    if (s instanceof Promise) { } // Clearly no side-effects here. No possible changes to s.
    s // s is Set<number> | (Set<number> & Promise<any>)                             <===== (2a)
    s.add(42); // No error this time                                                 <===== (2b)
}

function g(s: Set<string> | Set<number>) {

    // (3)
    s // s is Set<string> | Set<number>
    if (s instanceof Set) {
        s // s is Set<string> | Set<number>
    }
    else {
        s // s is never                                                              <===== (3a)
        s.add(42); // ERROR 'add' does not exist on never                            <===== (3b)
    }

    // (4)
    s = new Set<number>();
    s // s is Set<number>
    if (s instanceof Set) {
        s // s is Set<any>
    }
    else {
        s // s is Set<number>                                                        <===== (4a)
        s.add(42); // no error, but we couldn't possibly have a Set instance here    <===== (4b)
    }
}

Expected behavior:

• inferred type of s at (1a) is Set<number>
• inferred type of s at (2a) is Set<number>
• inferred type of s at (3a) is never
• inferred type of s at (4a) is never
• Compiler errors at (3b) and (4b)
• No compiler errors at (1b) and (2b)

Actual behavior:

• inferred type of s at (1a) is Set<number> | Set<any>
• inferred type of s at (2a) is Set<number> | (Set<number> & Promise<any>)
• inferred type of s at (3a) is never
• inferred type of s at (4a) is Set<number>
• Compiler errors at (1b) and (3b)
• No compiler errors at (2b) and (4b)

Notes:

Brought over from #9862 which is closed. See that issue for the discussion leading to the above examples. Cc: Anders Hejlsberg (@ahejlsberg), Nathan Shively-Sanders (@sandersn)

[yortus]

Just tried the nightly - works great! Thanks for the brilliant work Anders Hejlsberg (@ahejlsberg)