Likely to cause new Flow errors:

• InTypes First mode, exported classes and functions cannot be reassigned.
• Added similar checking ofsymbol in non-strict equality as other primitive types (example). We allow for non-strict equality checks between two symbol types, and between a symbol and null/void (example).
• We now write properties to unsealed objects recorded during speculative checking, after speculation has succeeded.
• Added more complete handling for indexers in spreads:
  • Use the indexer on the right, when the left is empty, exact and has no indexer. [example]
  • Allow matching indexers to be spread. [example]
• Removed thenon-array-spread lint, and replaced it with a new .flowconfig option,babel_loose_array_spread.

Notable bug fixes:

• Improved union optimization for non-enum union equality checks.
• Fixed a bug in which$Exact<T> failed whenT resolved to aShape<T>. [example]

Misc:

• Improved performance of calculating the set of files that need to be checked.
• Cached the typed AST as part of the persistent connection client info, to speed up requests coming from an LSP client (DefinitionRequest, Hover, TypeCoverage).
• Improved dependency graph construction times, by using Hashtbl and mutable back edges.
• Improvedtool add-comments by
  • usingflowlint-next-line instead of adding full$FlowFixMe suppressions on lints, and
  • making the rule for inserting comments inside JSX elements also apply to JSX fragments.
• Upgraded to sedlex 2.1.