Inferred return type for function that return type assertion should be `is [type]`

[mohsen1]

TypeScript Version: 2.2.1

Code

function isString(x: any) {
    return typeof x === 'string';
}

Expected behavior:

Inferred type of foo is function isString(x: any): boolean

Actual behavior:

Inferred type should be function isString(x: any): x is string

[mhegazy]

Related to #5101 (and #10734 tangentially)

I believe we discussed this before in the past, and concluded it was too complex to implement in the general case. doing it right needs to combine control-flow analysis with some sort of data flow analysis.