[typescript] Fix theme overrides type failing

[bopfer]

Closes #1858

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[cypress]

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Test summary

78 • 0 • 1 • 0

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Run details

Project	material-ui-pickers
Status	Passed
Commit	fe0930a
Started	Jun 9, 2020 3:37 PM
Ended	Jun 9, 2020 3:39 PM
Duration	01:30 💡
OS	Linux Debian - 10.0
Browser	Chrome 80

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[oliviertassinari]

@bopfer Awesome, thanks for opening a standalone pull request.

It appears that something is wrong with the CI on the pickers repository. If I apply this diff in the main repository:

diff --git a/packages/material-ui/src/styles/overrides.d.ts b/packages/material-ui/src/styles/overrides.d.ts
index fe4dab79e..707323f0e 100644
--- a/packages/material-ui/src/styles/overrides.d.ts
+++ b/packages/material-ui/src/styles/overrides.d.ts
@@ -144,7 +144,7 @@ export interface ComponentNameToClassKey {
   MuiExpansionPanelDetails: ExpansionPanelDetailsClassKey;
   MuiExpansionPanelSummary: ExpansionPanelSummaryClassKey;
   MuiFab: FabClassKey;
-  MuiFilledInput: FilledInputClassKey;
+  MuiFilledInput?: FilledInputClassKey;
   MuiFormControl: FormControlClassKey;
   MuiFormControlLabel: FormControlLabelClassKey;
   MuiFormGroup: FormGroupClassKey;

and run yarn typescript, I get the following error:

' is not assignable to type 'string'.
    Type 'undefined' is not assignable to type 'string'.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

the same one that was reported on the linked issue: #1858.
This sounds like a new argument in favor of increasing the priority of mui/material-ui#19706.

Do we have any idea on how we could update the CI in the pickers to catch the problem in the first place? It could be interesting to figure out in order to further align pickers with the main repo, I would fear we either introduce regression until them migration is completed and that we have other similar cases like this on in the pickers repository.

[dmtrKovalenko]

I am not sure why the problem appears – our tests are telling us that everything works fine. Just checked it with up-to-date next https://github.com/mui-org/material-ui-pickers/blob/6cde0eaf11531d10667d9f4d78bbcb9b3d51763f/lib/src/__tests__/typescript/Overrides.tsx#L4

[oliviertassinari]

Yeah, I think that there is an issue with how the TypeScript runs in the pickers repository. I came to this conclusion after looking at the behavior on the main repository and the reproduction example.

Maybe it has something to do with the options we give to TypeScript here, and how strict we are with the soundness?

[dmtrKovalenko]

our version is v3.9.3 and everything is as strict as possible.
Needs to investigate