☂️ Known unsoundness issues

[SamChou19815]

We have a lot of reports on unsoundness issues. This issue categorizes them into the underlying cause.

• Unsound literal types
  • Default value is not validated when deconstruction is used #8972
  • Inferred array type allows unsound covariance? #8607
  • Possibility to call non-existing method on array element without explicit 'any' #8593
  • Flow infers inconsistent types on mutable array #8108
  • typeof is unsound with string literals #7961
  • Flow allows unsound object type assertion #7360
  • Unsoundness when mutable objects are used (false has type true) #6893
  • Flow ignores literal property update #3630
  • incorrect type inference with Array on function parameters #3303
• Unchecked dictionary object creation with string
  • An object-as-map's value type is ignored when returning the object from a function #8719
  • False positive with simple case using bracket notation in objects #8426
  • Defined property type is not used if indexer type is an object property #8409
  • A safe cast to string makes Flow lose track of types? #8122
  • flow fails to report error for map key value structure when dynamic keys are used and the key argument is typed #7258
  • Computed properties - expecting a flow error, but there is none. #2928
  • Admits object literals with incompatible types to map when using computed properties #1971
• Unchecked variance in interfaces
  • Unsoundness with covariant type #8718
  • def-site variance does not prohibit illegal method declarations for interfaces #4287
• Unchecked variance in builtin utility types
  • $PropertyType lookups on class parameters allows variance abuse to sneak through class method checking #7830
  • $Diff<A, B> causes property variance in A to be lost in resulting type #6225
• Unsound generics
  • Spreading a generic value allows adding unexpected properties to it #8745
  • Excess property on polymorphic type with upper bound is unsafe #6661
• Object.freeze unsoundness
  • $Frozen<Type> object type #8842
  • Object.freeze subtyping is unsound #7964
• Class field initialization check
  • Non initialize class fields doesn't treats as nullable #8588