Likely to cause new Flow errors:

• We now require all generic functions to be fully annotated to preventgeneric-escape issues. Some escaped-generic errors will be removed withmissing-local-annot errors on return type annotation positions.
• Under LTI, we will no longer silently give unannotated parameters in destructuring assignmentany type. Instead, they will be properly contextually typed. e.g.try Flow example.
• Some additional errors might be revealed after a correctness fix in implicit instantiation. e.g.try Flow example
• Theexact_by_default option is now required to be set in the[options] section of the.flowconfig. Add eitherexact_by_default=true orexact_by_default=false to your[options] section. Previously the absence of the option was equivalent toexact_by_default=false. In the future, we will make the absence of the option equivalent toexact_by_default=true. Read more about the option here:https://flow.org/en/docs/config/options/#toc-exact-by-default-boolean. To create a valid new.flowconfig, useflow init.
• Changes to type variable unification may cause new errors to surface or old errors to be shifted to new locations.
• Fixed a bug where an unannotated parameter does not cause amissing-local-annot error under LTI. e.g.try Flow example
• Flow will stop emitting some spurious errors and uncover some hidden bugs under local type inference, as a result of removing a cache that no longer makes sense.
• Thethis type of exported classes' methods are no longer unsoundly typed asany. As a result, more underconstrained implicit instantiation errors might show up in LTI.
• Relational comparisons between dates and numbers are no longer allowed.

New Features:

• We addedflow autofix missing-local-annot command that will try to add annotations to autofix allmissing-local-annot errors in a given file.
• We now provide autofixes in IDE for allmissing-local-annot errors where we can locally infer a type. Previously, this kind of autofix is only provided for unannotated parameters.
• Added anunused-promise-in-sync-scope lint which will detect unused promises in sync scopes (likeunused-promise-in-async-scope does for async scopes). Enablingunused-promise will enable both of these rules. The individual rules will be removed in a future release.
• Pragmas (e.g.// @flow strict,// @jsx custom_jsx_fun) are now supported in try-flow.

IDE:

• Added autocomplete for jsdoc comments on functions.
• Added autocomplete for$FlowFixMe comments.
• Autocomplete will no longer suggest existing type names for type binding identifiers (e.g., in the name of a type alias).

Notable bug fixes:

• We will no longer warn aboutmissing-local-annot when there is a proper typing context insuper call. e.g.super(x => x) will now pass type checking under LTI if the super constructor call expects such function.
• Builtin types like$ReadOnly will no longer shadow your local definitions with the same name.
• We will no longer emit spurious errors when we are doing contextual typing while trying to resolve an overload. e.g.try Flow example
• We fixed a bug that over-zealously uses contextual type information from sibling nodes to type generic function calls. As a result, examples like the following no longer error:try Flow example
• fixed a bug where optional chaining on a value typed as a type application would not filter out the null or undefined value. e.g.try Flow example
• Some spurious errors might be removed as a result of Flow doing proper cache invalidation.
• Fix sorting of auto-imports with the same similarity score and length.

Parser:

• Parse and error on JSX type arguments.
• Add custom parse error for abstract classes.
• Add custom parse error when attempting to use template literal types in Flow.
• Add custom parse errors for TS class visibility modifiers public/private/protected.
• Removefunction as an alias toFunction, and don't allowfunction as a type name in general. (closesissue #9000).