[typescript] Fix withStyles edge cases, require TS 2.8

[pelotom]

Fixes #11109.
Fixes #11191.

This employs conditional types to define an Overwrite type operator which resolves the edge cases around withStyles, without destroying the integrity of props that are specified as a union. This means requiring a minimum TypeScript version of 2.8.

[estaub]

I took it around the block with all of my own code. All looks good!

You might want to document Overwrite a bit; it takes some staring at it and examples to understand. Something like:

Overwrite takes a type T and a "mixin" type U, removes any properties from T that would conflict with properties in U, and returns the combined type T&U. When T is equivalent to U, it just returns T.

[pelotom]

It seems like the stakeholders from the issues that this resolves are all in favor of it, so I'll merge. We'll want to mention in the release notes that TypeScript 2.8 is required now.

[oliviertassinari]

@pelotom Oops, just after the release train 🚂.

[pelotom]

@oliviertassinari 😩

[oliviertassinari]

The next one will be this weekend :)

[semiadam]

@pelotom will this fix all the compile errors introduced around withStyle after beta.43? My code had a comfortable life before 43 :)

[pelotom]

@semiadam with any luck 🙂

[olee]

@semiadam you can import the following fix instead of material-ui/styles to get around the compile errors:

import withStylesOld from 'material-ui/styles/withStyles';

export * from 'material-ui/styles';

import { WithStyles, StyleRules, StyleRulesCallback, StyledComponentProps } from 'material-ui/styles';
import { WithStylesOptions } from 'material-ui/styles/withStyles';

type ConsistentWith<O> = Partial<O> & Record<string, any>;
declare function WithStylesFixed<ClassKey extends string>(
    style: StyleRules<ClassKey> | StyleRulesCallback<ClassKey>,
    options?: WithStylesOptions,
): {
    <P extends ConsistentWith<StyledComponentProps<ClassKey>>>
    (component: React.ComponentType<P & WithStyles<ClassKey>>): React.ComponentType<P & StyledComponentProps<ClassKey>>;
    (component: React.ComponentType<WithStyles<ClassKey>>): React.ComponentType<StyledComponentProps<ClassKey>>;
};

export const withStyles =  withStylesOld as typeof WithStylesFixed;

[estaub]

@olee Do you know of some problem with @pelotom's fix for this? IIt might be more productive if folks tried that. Flipping the overloads just causes problems in other cases. If you want to see, try patching a copy of material-ui with your declaration, npm run typescript, and see what breaks.

[estaub]

@pelotom In your copious spare time, you might want to consider adding test cases for the issues that were posted due to my PR, either in this PR or another.

[pelotom]

@pelotom In your copious spare time, you might want to consider adding test cases for the issues that were posted due to my PR, either in this PR or another.

I did, in this PR... were there others that I missed?

[estaub]

@pelotom Sorry, no need for more AFAIK. I looked for them in the diffs, don't understand how I missed them!