bug(input): matInput overwrites aria-described by

[josephperrott]

Copied from internal bug report:

According to https://material.angular.io/components/input/overview#accessibility:

Any mat-error and mat-hint are automatically added to the input's aria-describedby list,...

but it seems they are not added but replace the pre-existing aria-describedby, see here

We use the aria-describedby to announce keyboard shortcuts for sending the form, and that breaks if matInput overwrites this.

Would it be possible to store the previous aria-describedby attribute and maintain it whenever setting this? That seems more inline with the docs and allows e.g. our use case.

cc: @rehmsen

[devversion]

That's an interesting bug. I agree that this behavior is confusing. We could get around it by merging it with the static aria-describedby attribute value that we inject in the MatInput constructor.

[rehmsen]

Thanks for filing it here.

Another suggestion that was proposed internally is for the error handling to use aria-errormessage instead of aria-describedby. Example:

<label>
  Comment
  <input matInput type="text" value="???" aria-errormessage="error">
</label>
<p id="error">
Please enter a valid comment
</p>

Would have to check how well this is supported by screenreaders though, and we might still not want to clobber the value passed by the component user.

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