chore(distinctUntilKeyChanged): nail down key's typing with keyof T

[imcotton]

Description:

With better typing constraint on T[key] looks up from TypeScript.

[ci-reporter]

The build is failing

✨ Good work on this PR so far! ✨ Unfortunately, the Travis CI build is failing as of 0a0c9f4. Here's the output:

npm test

> @reactivex/rxjs@6.2.2 test /home/travis/build/ReactiveX/rxjs
> cross-env TS_NODE_PROJECT=spec/tsconfig.json mocha --opts spec/support/default.opts "spec/**/*-spec.ts"


/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:250
    return new TSError(diagnosticText, diagnosticCodes)
           ^
TSError: ⨯ Unable to compile TypeScript:
spec/operators/distinctUntilKeyChanged-spec.ts(209,68): error TS2345: Argument of type '(x: number, y: number) => boolean' is not assignable to parameter of type '<K>(x: K, y: K) => boolean'.
  Types of parameters 'x' and 'x' are incompatible.
    Type 'K' is not assignable to type 'number'.
spec/operators/distinctUntilKeyChanged-spec.ts(225,68): error TS2345: Argument of type '(x: number, y: number) => boolean' is not assignable to parameter of type '<K>(x: K, y: K) => boolean'.
  Types of parameters 'x' and 'x' are incompatible.
    Type 'K' is not assignable to type 'number'.

    at createTSError (/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:250:12)
    at getOutput (/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:358:40)
    at Object.compile (/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:545:11)
    at Module.m._compile (/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:430:43)
    at Module._extensions..js (module.js:663:10)
    at Object.require.extensions.(anonymous function) [as .ts] (/home/travis/build/ReactiveX/rxjs/node_modules/ts-node/src/index.ts:433:12)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)
    at Module.require (module.js:596:17)
    at require (internal/module.js:11:18)
    at /home/travis/build/ReactiveX/rxjs/node_modules/mocha/lib/mocha.js:231:27
    at Array.forEach (<anonymous>)
    at Mocha.loadFiles (/home/travis/build/ReactiveX/rxjs/node_modules/mocha/lib/mocha.js:228:14)
    at Mocha.run (/home/travis/build/ReactiveX/rxjs/node_modules/mocha/lib/mocha.js:536:10)
    at Object.<anonymous> (/home/travis/build/ReactiveX/rxjs/node_modules/mocha/bin/_mocha:582:18)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)
    at Function.Module.runMain (module.js:693:10)
    at startup (bootstrap_node.js:191:16)
    at bootstrap_node.js:612:3

I'm sure you can fix it! If you need help, don't hesitate to ask a maintainer of the project!

---

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[coveralls]

Coverage remained the same at 96.951% when pulling bc9768d on imcotton:nail-down-distinctUntilKeyChanged-key-type into bb77e25 on ReactiveX:master.

[cartant]

Thanks for the PR.

LGTM, but could you add a couple of dtslint-based tests?

I'd like all typings changes to be accompanied by dtslint-based tests - so that we can build out the coverage.

In particular, I'd like to see a test that expects an error if the K is not a key of T - to guard against a regression.

For an example, have a look at https://github.com/ReactiveX/rxjs/blob/master/spec-dtslint/operators/concatAll-spec.ts

To run the dtslint tests, use this command:

npm run dtslint

[imcotton]

Hi, thanks for the guidance of dtslint, I've add a couple of cases into its dtslint spec, hopefully it covers everything, feel free to pointing out if need further improvements.

[cartant]

LGTM