TypeScript errors upgrading MUI from 4.9.5 to 4.9.9

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

I have been able to trace the following errors to some changes made recently to use TS features such as Omit which aren’t available in 3.2.4 (as far as I understand, that’s the minimum TS version that MUI supports) or trailing commas:

• https://github.com/mui-org/material-ui/pull/20264 (Omit)
• https://github.com/mui-org/material-ui/pull/20211 (the trailing comma issue)

Click to expand

node_modules/@material-ui/core/Backdrop/Backdrop.d.ts:8:52 - error TS2304: Cannot find name 'Omit'.

8     React.HTMLAttributes<HTMLDivElement> & Partial<Omit<FadeProps, 'children'>>,
                                                     ~~~~

node_modules/@material-ui/core/Box/Box.d.ts:29:22 - error TS1009: Trailing comma not allowed.

29     typeof typography,
                        ~

node_modules/@material-ui/core/Fade/Fade.d.ts:4:36 - error TS2304: Cannot find name 'Omit'.

4 export interface FadeProps extends Omit<TransitionProps, 'children'> {
                                     ~~~~

node_modules/@material-ui/core/TextareaAutosize/TextareaAutosize.d.ts:4:11 - error TS2304: Cannot find name 'Omit'.

4   extends Omit<React.TextareaHTMLAttributes<HTMLTextAreaElement>, 'children' | 'rows'> {
            ~~~~

node_modules/@material-ui/core/styles/shadows.d.ts:26:9 - error TS1009: Trailing comma not allowed.

26   string,
           ~

Expected Behavior 🤔

Steps to Reproduce 🕹

Steps:

1. Install @material-ui/core@4.9.9
2. Run your project through tsc using typescript@3.2.4
3. Observe the errors above

Context 🔦

We set a minimum supported version of TypeScript to 3.2.4 in our design system to match Material-UI but seems like the latest MUI version uses newer TypeScript features. This isn’t usually a big issue to React projects, but we also support Angular teams via Custom Elements and usually bumping TypeScript requires them to bump their Angular versions too (usually across major version boundaries) which can be a bit painful. For example, Angular 7.2 supports TS 3.2.4, Angular 8.0.0 adds support for 3.4 dropping support for older versions, Angular 9.0.0 requires 3.6/3.7

Your Environment 🌎

Tech	Version
Material-UI	v4.9.9
TypeScript	v3.2.4

[eps1lon]

Yeah this is what we wanted to avoid but I’d argue that this is more of an issue with a PR that was too big not the tooling.

I would rather move forward here though (upcoming @ts-expect-error and tslint -> eslint). Usage numbers for earlier typescript version is pretty low so I don’t think it’s justified to build tooling for 3.2. But this is for me personally. If wants to build it I’m all ears.

[oliviertassinari]

@eps1lon I wanted to ask about this a couple of days ago, this is an opportunity. What do you think about this diff?

diff --git a/docs/src/pages/getting-started/supported-platforms/supported-platforms.md b/docs/src/pages/getting-started/supported-platforms/supported-platforms.md
index befc4ca39..e49aeba94 100644
--- a/docs/src/pages/getting-started/supported-platforms/supported-platforms.md
+++ b/docs/src/pages/getting-started/supported-platforms/supported-platforms.md
@@ -36,3 +36,9 @@ It's a must-do for static pages, but it needs to be put in balance with not doin

 Material-UI supports the most recent versions of React, starting with ^16.8.0 (the one with the hooks).
 Have a look at the older [versions](https://material-ui.com/versions/) for backward compatibility.
+
+## TypeScript
+
+Material-UI supports the most recent versions of TypeScript.
+However, the codebase doesn't follow semver for types.
+You can expect breaking changes between two minor versions.
+As of now, we support TypeScript ^3.8.0.
+Have a look at the older [versions](https://material-ui.com/versions/) for backward compatibility.
diff --git a/packages/material-ui/package.json b/packages/material-ui/package.json
index e682bb751..0b43f57b5 100644
--- a/packages/material-ui/package.json
+++ b/packages/material-ui/package.json
@@ -39,9 +39,13 @@
   "peerDependencies": {
     "@types/react": "^16.8.6",
     "react": "^16.8.0",
-    "react-dom": "^16.8.0"
+    "react-dom": "^16.8.0",
+    "typescript": "^3.8.0"
   },
   "peerDependenciesMeta": {
+    "typescript": {
+      "optional": true
+    },
     "@types/react": {
       "optional": true
     }