type for Function.prototype.bind wrongly preserves fields on a function

When calling bind on a function Javascript does not copy the properties across that have been added to that function, however TS treats them as having been copied.

TypeScript Version: 3.7-Beta

Search Terms: function bind

Code

const a = () => { }
a.hello = 'hi'

const b = a.bind({})

b.hello.split("") // Uncaught TypeError: Cannot read property 'split' of undefined

Expected behavior: hello field should not be present on typeof b.

Actual behavior: hello is deemed to exist on typeof b and the program crashes.

Playground Link: http://www.typescriptlang.org/play/?ts=3.7-Beta&ssl=1&ssc=1&pln=7&pc=1#code/MYewdgzgLgBAhjAvDAFASiQPhgbxgXwCg4A6ACwFMAbKkJGAcjIEsHDDRJYAje075mAAmKHPjTtu5arRIQADlWZQUAIlUYA9JpgBVMMDgBXAOZlYAFQCe8igFEATg5AOAXDADCcMGBCwHFHBCMPLOtg5QVowKSlAMMCAAZjBGwhSJghRChEA

Related Issues: https://github.com/Microsoft/TypeScript/issues/212 seems to be where this was discussed and finally implemented, although it's not directly related.

Commencez par le lien vers TypeScript Playground et reproduisez l’exemple de bind présenté avec la version de TypeScript indiquée. Comparez le type inféré de b avec le comportement à l’exécution de JavaScript ; le travail est terminé lorsque le champ hello ajouté est rejeté pour typeof b, tandis que le comportement valide de bind reste pris en charge.