IDE:

• For React autoimport, it will now always be in the form of import * as React from 'react'

Misc:

• Minor changes to the flow-upgrade script

Parser:

• Support export declare, like declare export
• For component in implicit declare context, the ast we produce will include implicitDeclare: true

Likely to cause new Flow errors:

• Imported functions that conform to hook naming conventions will always be treated like hooks. As a result, new errors might be surfaced. e.g. conditional hook call

Parser:

• flow_parser.js now allows you to pass in filename in the option object

New Features:

• declare keyword can be omitted for declare function under declaration context (.flow file, declare module, declare namespace)

Parser:

• Fix parsing of experimental record expressions that are multiline

Declarations:

• The declare keyword can be omitted for declare const/let/var variable: type and declare component under a declaration context (libdef file, .js.flow file, declare module, declare namespace)
• declare component now requires the rest props to be named - matching normal component declarations

Misc:

• The Omit<O, K> utility type will no longer enforce that the provided keys K need to be part of O

New Features:

• Support 4 well-known symbols (Symbol.iterator, Symbol.asyncIterator, Symbol.dispose, Symbol.asyncDispose) in class methods. For now, the computed method name needs to be exactly in the form of Symbol.iterator (or other 3 names) to be recognized.

Notable bug fixes:

• Flow will now error when a function parameter appears in the default expression for the same binding. (e.g. try-Flow)

Misc:

• Introduced a codemod for migrating legacy utility types to modern equivalents (yarn run flow-codemod convertLegacyUtilityTypes)

Parser:

• Flow now parses import attributes. For now, we don't do any semantic validation yet

Parser:

• Support second argument of import expressions

Bug Fixes:

• Fixed bug in computing type hints under JSX expressions (Example of bug fixed)

Config:

• use_mixed_in_catch_variables option was renamed use_unknown_in_catch_variables because mixed is being renamed to unknown