v0.21.0

Likely to cause new Flow errors:

• ES6 react classes without state should now extends React.Component<DefaultProps, Props, void> (previously it was extends React.Component<DefaultProps, Props, {}>)
• ES6 react classes with state should declare state: State;

New features:

• Autocomplete for jsx properties
• Typed JSX intrinsics. This means you can list which jsx intrinsics exist (like div, span, etc) and specify which properties they have.
• Syntax for declaring variance at definition. For example, interface Generator<+Yield,+Return,-Next> {...}. Still pending transpiler support though.
• Refining string and union types with string equality now properly refines the types.
• Support for export * as from @leebyron's Stage1 proposal. Babel support here

Notable bug fixes:

• Fixed bug with class expressions due to this type
• Fixed autocomplete for this
• Recognizes exhaustiveness in switch statements with default case.
• Fixed "Did not expect BoundT" errors
• Fixed infinite loop in certain recursive types
• Fixed an incremental mode issue with deleted files
• Fixed an incorrect refinement when assigning an object to a variable.

Misc:

• Some internal errors now will be made user visibile instead of silently failing. They generally mean that Flow has some bug or is making an untrue assumption/assertion. If you see these please report them!
• Improvements to how we report certain types (type application, optional types) via our APIs
• Various sentinel improvements, including boolean sentinels
• Various improvements to the buildin flow libraries (thanks everyone for the pull requests!)