Merge compiler options deep

[d4rkr00t]

Hi,
It's probably more conversation starter then actual PR. If you find it useful/appropriate I'll add tests.

The problem:

Imagine that we have such tsconfig.json:

{
    "extends": "./../tsconfig.base",
    "compilerOptions": {
        "declaration": true,
        "lib": ["es2015", "es2016"]
    },
    "include": [ "*.ts" ]
}

And tsconfig.base.json is:

{
    "compilerOptions": {
        "noUnusedLocals": true,
        "noImplicitAny": true,
        "sourceMap": true,
        "target": "es6",
        "lib": ["es5", "dom"]
    }
}

As a result of extending base config we will get compilerOptions which looks like that:

{
    "compilerOptions": {
        "declaration": true,
        "noUnusedLocals": true,
        "noImplicitAny": true,
        "sourceMap": true,
        "target": "es6",
        "lib": ["es2015", "es2016"]
    }
}

As you can see, "lib" now only contains items from tsconfig.json. It means we can't add extra items to libs, we can only completely redeclare them. And i'm not sure if it was intended or not.

The only bad thing with merging "lib" i can think of is it becomes impossible to redeclare them. But probably it's not an issue.

What do you think?

[DanielRosenwasser]

Well, when we first discussed the idea of the config inheritance (i.e. extends fields in tsconfig.json), there wasn't much discussion, but the decisions were largely unanimous to take the simpler option between merging and overwriting. Check out #10566.

[d4rkr00t]

Yeah, sorry for such late response :) Probably i should close this PR :)