Decide how to reconcile `async` pipe with strict template type checking

🚀 feature request

Relevant Package

This feature request is for @angular/common

Description

Currently the async pipe is difficult to use with strictTemplates and strictNullChecks enabled. This is because the async pipe emits null if the underlying observable has not emitted yet. Therefore using the async pipe with an input that does not accept null will result in a compilation error.

@devversion has done a more detailed write-up of this issue. I’m including a summary of the possible solutions here, but please refer to the write-up for more details.

Describe the solution you’d like

The ideal solution would be to make async pipe not emit until the underlying observable emits, however this would likely be a fairly breaking change. It would probably need to be done in a phased manner, while introducing something like asyncEager that maintains the old behavior to make migration easier.

Describe alternatives you’ve considered

1. An option that would split the responsibility for fixing between framework and libraries is to introduce something like @RequiredInput(fallbackValue) that automatically replaces null with fallbackValue. Libraries could then update their inputs to use @RequiredInput when null is not expected.

2. Another option that would place the responsibility on libraries is to ask them to just handle null and allow it to be passed in the template even if its not part of the input’s official type (using static ngAcceptInputType_*)

3. Finally, a third alternative is to just pass the responsibility down to the app developer to fill in a default: [someInput]="(value$ | async) || defaultValue"

[IgorMinar]

we discussed this today, and we agreed that to fix this properly, we should look at typing of inputs in general and properly allow inputs to be optional, required, etc.

for v9 the best we can likely do is that for users that want to use strictTemplates and strictNullChecks together, they can opt out of strict null typecheck specifically for input bindings via strictNullInputTypes : https://v9.angular.io/guide/template-typecheck#troubleshooting-template-errors

This should be explicitly documented in the v9 updating guide or template type checking guide.

[alxhub]

I consider this resolved. With the way Angular works today, at runtime the async pipe will set a null value into inputs. I think that strictTemplates should be enforcing that this is possible, unless you explicitly opt out (strictNullInputTypes: false).