Correct types for functions stored in useState

[totalolage]

Summary

Using a function initialiser to a useState hook which was typed to take a function as state could result in the value not being typed as optional where in reality it is.

Example:

// No TS error despite this not being correct. Should error with `void is not assignable to ()=>void`
// instead `value` is typed as `()=>void` but actually has the value `undefined`.
const [value, setValue] = useState<()=>void>(() => {}); 

How did you test this change?

I used the following to validate that my type NotFunction detects all function types (both objects with call signatures and Function instances) correctly: typescript playground link

I validated that the erroneous type assignment is fixed by my changes in this ts playground.

[react-sizebot]

Comparing: 5d4b758...4d951ea

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	177.14 kB	177.14 kB	=	55.07 kB	55.07 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	172.98 kB	172.98 kB	=	53.93 kB	53.93 kB
facebook-www/ReactDOM-prod.classic.js	=	592.80 kB	592.80 kB	=	103.99 kB	103.99 kB
facebook-www/ReactDOM-prod.modern.js	=	574.49 kB	574.49 kB	=	101.02 kB	101.02 kB
test_utils/ReactAllWarnings.js	Deleted	64.93 kB	0.00 kB	Deleted	16.24 kB	0.00 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
test_utils/ReactAllWarnings.js	Deleted	64.93 kB	0.00 kB	Deleted	16.24 kB	0.00 kB

Generated by 🚫 dangerJS against 4d951ea