Packages should define what minbar TypeScript is supported

[dzearing]

Problem:

Customer uses TypeScript 3.7 with Fluent UI and starts getting syntax errors because TypeScript doesn't recognize newer syntaxes in d.ts files.

Proposed solution (one of many possibly):

In package.json for each package, specify types as unsupported.d.ts. Additionally use typesVersions entry to specify the minbar TypeScript version:

{
  "types": "unsupported-typescript-version.d.ts",
  "typesVersions": {
    ">3.9": "lib/index.d.ts"
  }
}

See documentation here: https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html#version-selection-with-typesversions

Versions which don't resolve typings in typesVersions will fail to pick up typings, which is a better error than random syntax errors in various places.

Tools can also use the typesVersions entries to make decisions; for example, my recent codemod work of moving to import type syntax could special case packages that support < TS 3.9 if this information were programmatically accessible from the package definition.

Alternative solution:

Use typesVersions to redirect older versions to unsupported.d.ts. This will work most of the time, and maybe require less modifications in the future. This will work with TypeScript 3.1 and newer, but for older versions than that it won't be respected while the suggested solution should work for very very old versions.

[msft-fluent-ui-bot]

Because this issue has not had activity for over 180 days, we're automatically closing it for house-keeping purposes.

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