Likely to cause new Flow errors:

• Array literals that cannot be contextually typed can no longer be passed to mutable tuples with a more general element type. It might cause additional errors. example

Likely to cause new Flow errors:

• Hook calls inside anonynous functions bound to a variable will get react-rule-hook-definitely-not-in-component-or-hook error, if the variable name doesn't conform to hook naming convention. example

IDE:

• Added support for workspace symbol feature

Misc:

• Thanks @jbroma for improving as casts with function generics and component generics!

Likely to cause new Flow errors:

• For all object literals in positions that cannot be contextually typed, we will infer a stricter type for them. It will cause new errors in code like

const foo = {baz: new Dog()};
type Foo = {bar?: string, baz: Animal};
declare function acceptFoo(foo: Foo): void;
acceptFoo(foo); // error

To fix the error, you can either annotate the object

const foo: Foo = {baz: new Dog()};
type Foo = {bar?: string, baz: Animal};
declare function acceptFoo(foo: Foo): void;
acceptFoo(foo);

or make the call site accepts readonly objects:

const foo = {baz: new Dog()};
type Foo = $ReadOnly<{bar?: string, baz: Animal}>;
declare function acceptFoo(foo: Foo): void;
acceptFoo(foo);

We provide a codemod to automate the annotation process. flow codemod annotate-literal-declaration --write --max-type-size 5. (You can adjust the max type size based on your needs).

IDE:

• Support rename on private properties and methods.

Library Definitions:

• React$MixedElement is removed from builtin libdef. It will cause internal-type error since v0.258.0. You should use React.MixedElement instead.

• Bug fixes for match

New Features:

• Support for experimental match feature using option experimental.pattern_matching=true

Likely to cause new Flow errors:

• Unannotated object literals reachable from exports will now be inferred to have all mutable fields when being imported. Previously, it has unsound types, so new errors might appear.
• When comparing two object types whose properties have variance incompatibilities, Flow will raise a single error that will summarize the properties with incompatible variances, instead of a single error for each property. (e.g. try-Flow)
• When an object with extra properties is passed to a place that expect an exact object, we will now generate a single error with all extra properties. The error message will list the extra properties, and state that "Exact objects do not accept extra props". In rare cases, the error locations might be moved.
• Flow will error more consistently with sketchy-bool on nullable boolean types (e.g. try-Flow)

Library Definitions:

• All properties in the builtin PropertyDescriptor type are marked as readonly. If you need a mutable version, you can introduce something like type MutablePropertyDescriptor<T> = {...$Exact<PropertyDescriptor<T>>, ...}

Notable bug fixes:

• Fixed windows builds.
• Fixed crash when loading saved state.

Likely to cause new Flow errors:

• We are announcing Natural Inference for Flow, an improved way to infer types for primitive values, that resolves a long-standing correctness gap and source of confusion. See more in this post.
• Added nested-hook and nested-component lint errors which detect nested hook or component syntax within component or hook syntax. This is on by default.

Notable bug fixes:

• For default imports, the autoimport ranking will now consider the names of the importing side. (e.g. Previously we completely ignored the name of foo in import foo from './bar', but now we will count foo. If the pattern of import foo from './bar' happens a lot, then the autoimport algorithm will be more likely to suggest import foo from './bar' rather than import bar from './bar').
• Flow will infer a correct type when viewing the type of an object literal as a dictionary type. For example, the error in this try-Flow will be raised.
• Previously, we undercounted some imports during indexing, which causes autoimport ranking to behave incorrectly. The issue is now fixed.
• Flow will no longer emit react-rule-hook-conditional error for hooks called in a conditionally defined nested component.

Notable bug fixes:

• Prevent non-termination when computing code actions.

Notable bug fixes:

• Fixed https://flow.org/try. It was broken from v0.270.0 to v0.272.0.
• Documentation in hover now preserves indentations. Therefore, code blocks in jsdoc will no longer lose indentation.