Flow: Regression in v0.38.0 when checking object properties in conditionals

[STRML]

This appears related to #1879, but another bug was added in v0.38.0:

type ExactObj = {|
    a: string
|};

const someObj: ExactObj = {a: 'string'};
const test1 = someObj.COMPLAIN_PLZ; // expected error
if (someObj.COMPLAIN_PLZ) { // bug: should complain

}

// Errors in 0.37.4, not in 0.38.0 or master.
if (someObj.COMPLAIN_PLZ === true) { // bug: should complain. 

}

if (Boolean(someObj.COMPLAIN_PLZ)) { // expected error

}

Try it.

This special-casing of conditionals appears also related to https://github.com/facebook/flow/issues/3309.

I don't see why conditionals should have special rules; someObj.missingProperty should be an error regardless of where it's written, and we should be encouraging hasOwnProperty or in checks.

[vkurchatkin]

This is clearly not a bug or regresseion, it's even mentioned in the changelog:

We've relaxed sentinel checks to allow you to check unknown properties. This is consistent with other conditionals

[STRML]

My apologies, missed that; came up as it appears some refinement being done relates to #3309.

[vkurchatkin]

To be honest, I don't understand this change too

[minedeljkovic]

We've relaxed sentinel checks to allow you to check unknown properties. This is consistent with other conditionals

I cannot find a reason why would that be relaxed for Exact types.
It makes something like this fragile:

type BagOfFlags = {|
  a: boolean,
  b: boolean,
  // REMOVED c: boolean
|}

declare var bag: BagOfFlags;

if (bag.c) { // would be nice not to typecheck after c is REMOVED
  // ...
}

IMO, it should not be relaxed even for non exact types, since it leads to dangerous implicit any like in #3309 .
Can this be made configurable in flowconfig?

[vkurchatkin]

With exact objects you might want to do this:

type T = {| foo: string |} | {| bar: string |};
declare var a: T;
const x: ?string = a.foo;

[JunyuanZheng]

Is that possible to make this configurable? @vkurchatkin