facebook/flow

Likely to cause new Flow errors:

Several updates to Flow's understanding of React's APIs. Some of these updates remove old/deprecated React APIs.

New features:

Flow now gives precedence to library definitions over non-@flow implementation files. This means that it should no longer be necessary to specify a node_modules dependency in the [ignore] section of your .flowconfig if you have a library definition defined for that dependency.
Significant improvements to Promise.all: We now preserve the type of each item in the array passed to Promise.all() so that it may be propagated through to the resulting .then() handler.
We no longer try to parse files that are not marked with an @flow pragma. We anticipate this will improve performance for projects with large, non-Flow node_modules directories.
Classes with static members are now subtype-compatible with structural object types
It is now possible to specify a leading | or & for type aliases of long unions/intersections. This is useful, as one example, for disjoint unions with a large number of members (where each member sits on a new line):

type MyDisjointUnion =
  | {type: 'TypeOne', ...}
  | {type: 'TypeTwo', ...}
  | {type: 'TypeThree', ...}
  ...
;

Bug fixes:

Fixed an issue where an intersection of two object types did not always properly combine to match objects with members on both sides (#1327)
Fixed an issue where an object of some intersection type could not be used with the spread operator (#1329)
Fixed an issue where refinement-testing on an object of an intersection type wouldn't always work (#1366)
Fixed an issue where an intersection of function types with a common return type should type check against function types with union of param types
Fixed an issue where refining obj['abc'] didn't behave quite the same as refining obj.abc
Fixed an issue where usage of flow get-def on the left-hand side of a require() wouldn't hop through the require() and to the actual location of the definition
Fixed an issue where Flow would not give a clear error when trying to use import type to get a non-type export
Fixed an issue flow dump-types --json was not as robust as it could be against outputting valid JSON in the event of certain kinds of errors
Fixed an issue where Flow would not give a parse error if multiple ES exports with the same name are exported from a single ES module
declare class declarations now properly define a built-in name property (like real class declarations do)