【flowtype/React/Class】これ。how to fix 「Covariant property `[fn]` incompatible with contravariant use in “` assignment of property `[fn]`」

【flowtype/React/Class】これ。how to fix 「Covariant property `[fn]` incompatible with contravariant use in ``` assignment of property `[fn]`」

flowtypeをReactに取り入れて、class構文で使おうとするとこのようなエラーが出る。

If you incorporate flowtype into React and try to use it in class syntax, you get such an error.

【flowtype/React/Class】これ。how to fix 「Covariant property `[fn]` incompatible with contravariant use in ``` assignment of property `[fn]`」

flowtype playground
https://github.com/kenmori/flow