bug: TypeScript target/lib option doesn't constraint ECMA features on type checking within packages

[Hotell]

Library

@fluentui/react - all v8 packages

Current Behaviour

While implementing #16976 I ran into following TS related(compile/runtime) bug within our packages.

• target/lib tsconfig option restriction has effect on code that is used within monorepo package

Expected behavior:

I should get TS errors based on target/lib setting.

Why is this happening?

The issues starts within react-conformance package

• because we are using ES2015 features, those 2 functions will infer return type based on node globals and thus will end up
  with following output
  
• node types are explicitly importing ECMA2015+ libs from TS standard lib thus adding those to TS eval scope
  • 

Now because this, any API that is used from this package will "extend" (pollute) TS eval scope of particular package where it's being used (in our example react-menu)

👉 That's why we don't see any errors in our react-menu (event though we are using ECMA2015+ features)

Let's say we'd fix consoleUtil.ts (implementing Object.assign on our own for example), the main problem would not go away.

Why?

• if we briefly skim through react-conformance implementation, we would discover that there is enzyme imported (within defaultTests.tsx
• now enzyme types add cheerio types (/// <reference types="cheerio" />) and cherio types add node types (/// <reference types="node" />) so we end up with the same issue

Conclusion
Now even if we remove enzyme and other related culprits, nothing will stop us to introduce such a anomaly in future, because we are mixing 2 environments (with storybook in place 3) within 1 TS config, those leaks like this are inevitable.

An elegant solution for this problem was already introduced in (this RFC), - tsconfig per environment consolidated via TS Solution style config.

Also the solution in more detail: https://gist.github.com/Hotell/19e776aa1a47258340b8bd9b477ad303

[gouttierre]

@Hotell - Thanks for the feedback and suggestion.

@khmakoto - would you be able to follow this one up ? I am not sure if you will need to include talks with @ecraig12345?

[Hotell]

What needs to be done:

• create RFC with proposed solution
• merge RFC
• prioritise and plan actual implementation in particular team with estimation
• execute implementation in particular milestone on 1 package ( react-menu is great candidate)
• implement migration script that every package can migrate to this new behaviour gradually
• set priority and milestone delivery per package with this new fixed behaviour