IThenable's (and thus goog.Promise's) non-success result cannot be expressed in Closure compiler's types

[mprobst]

IThenable. (from the common Thenable spec) is a promise for something of some type TYPE:

https://github.com/google/closure-compiler/blob/master/externs/es6.js#L704

But there's also the type that expresses an error, and the type that's passed to notify - ideally it'd be IThenable.<TYPE, REJECT, NOTIFY>. However that cannot be expressed in Closure's type system - the .then(fn(T): T2, fn(R): R2, fn(N): N2) method will return a new IThenable that should have new types inferred from the functions where the functions were passed in, i.e. not undefined (or null in practice).

AFAICT this cannot be expressed in Closure compiler, making promises harder to use, and migration to standard promises less promising (ha!).

[mprobst]

@cramforce might care about this?

[cramforce]

We left of the type of the error on purpose since in the general case it is impossible to anticipate that type. You may expect error X but there might also always be an NPE.