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Doug Parker fa835b5a29 feat(compiler-cli): enable extended diagnostics by default (#44712 )

Refs #42966.

Extended diagnostics provide additional analysis about Angular templates by emitting warnings for specific patterns known to be error prone or cause developer confusion. Currently, there are two such diagnostics which are enabled by default:

* `invalidBananaInBox` emits a warning if a user writes a two-way binding backwards like `([foo])="bar"`, when they actually wanted `[(foo)]="bar"`.
* `nullishCoalescingNotNullable` emits a warning if a binding attempts to perform nullish coalescing (`??`) on a type which does not include `null` or `undefined`, such as `{{ foo ?? 'bar' }}` where `foo` is defined as `string` instead of `string | null`.

These diagnostics are enabled as warnings by default, but this can be configured in the `tsconfig.json` like so:

```jsonc
{
  "angularCompilerOptions": {
    "extendedDiagnostics": {
      // The categories to use for specific diagnostics.
      "checks": {
        // Maps check name to its category.
        "invalidBananaInBox": "suppress"
      },

      // The category to use for any diagnostics not listed in `checks` above.
      "defaultCategory": "error"
    }
  }
}
```

Allowed categories for a diagnostic are `warning` (default), `error`, or `suppress`. `warning` emits the diagnostic but allows the compilation to succeed, `error` *will* fail the compilation, while `suppress` will ignore the diagnostic altogether.

The initial release has two diagnostics, and we are hoping to expand this longer term to add more diagnostics and provide additional insight into Angular templates to detect and surface developer mistakes *before* hours of debugging are wasted.

PR Close #44712

2022-01-19 09:58:37 -08:00