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This is a new feature of the Ivy TestBed.
A common user pattern is to test one component with another. This is
commonly done by creating a `TestFixture` component which exercises the
main component under test.
This pattern is more difficult if the component under test is declared in an
NgModule but not exported. In this case, overriding the module is necessary.
In g3 (and View Engine), it's possible to use an NgSummary to override the
recompilation of a component, and depend on its AOT compiled factory. The
way this is implemented, however, specifying a summary for a module
effectively overrides much of the TestBed's other behavior. For example, the
following is legal:
```typescript
TestBed.configureTestingModule({
declarations: [FooCmp, TestFixture],
imports: [FooModule],
aotSummaries: [FooModuleNgSummary],
});
```
Here, `FooCmp` is declared in both the testing module and in the imported
`FooModule`. However, because the summary is provided, `FooCmp` is not
compiled within the context of the testing module, but _is_ made available
for `TestFixture` to use, even if it wasn't originally exported from
`FooModule`.
This pattern breaks in Ivy - because summaries are a no-op, this amounts
to a true double declaration of `FooCmp` which raises an error.
Fixing this in user code is possible, but is difficult to do in an
automated or backwards compatible way. An alternative solution is
implemented in this PR.
This PR attempts to capture the user intent of the following previously
unsupported configuration:
```typescript
TestBed.configureTestingModule({
declarations: [FooCmp, TestFixture],
imports: [FooModule],
});
```
Note that this is the same as the configuration above in Ivy, as the
`aotSummaries` value provided has no effect.
The user intent here is interpreted as follows:
1) `FooCmp` is a pre-existing component that's being used in the test
(via import of `FooModule`). It may or may not be exported by this
module.
2) `FooCmp` should be part of the testing module's scope. That is, it
should be visible to `TestFixture`. This is because it's listed in
`declarations`.
This feature effectively makes the `TestBed` testing module special. It's
able to declare components without compiling them, if they're already
compiled (or configured to be compiled) in the imports. And crucially, the
behavior matches the first example with the summary, making Ivy backwards
compatible with View Engine for tests that use summaries.
PR Close #30578
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