☂️ 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 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