API: inconsistent Type#getProperties involving index signature and union type order

[ajafff]

TypeScript Version: 3.4.5, 3.5.0-dev.20190523

Search Terms:

Code

type IndexFirst = {[x: string]: string} | {x: string};
type IndexLast = {p: string} | {[p: string]: string};

Expected behavior:

type.getProperties() to return an array of one symbol for each of the type alias declarations.

Actual behavior:

For IndexFirst type.getProperties() returns an empty array.
For IndexLast type.getProperties() returns an array of one symbol.

AFAICT this is caused by an early break in getPropertiesOfUnionOrIntersectionType:

// The properties of a union type are those that are present in all constituent types, so
// we only need to check the properties of the first type

That's not quite correct if there are index signatures. Iterating through the rest of the union constituents would fix this. getPropertyOfUnionOrIntersectionType already strips properties not present in all constituents. So this is probably a performance optimization and shouldn't simply be removed. Maybe we could stop after the first constituent that has no index signature.

[RyanCavanaugh]

If there isn't a way to observe this during batch compilation or interactive scenarios, I would just call this a quirk of the API. It's not clearly a defect (i.e. wouldn't accept a PR to "fix" this with any negative perf implications).

[ajafff]

wouldn't accept a PR to "fix" this with any negative perf implications

@RyanCavanaugh
As you already mentioned this is not observable via batch compilation or language service. From the compilers POV this is dead code as it's never hit by any of the tests. For API users this function is not usable with this "quirk" as I cannot synthesize Symbols for union types.
Fixing this by iterating through all types with index signature will not have a perf impact since there's not way a user could ever notice it.

[ajafff]

Turns out this bug affects property access completions. I added a test case in PR #31979