Apply .mat-input-invalid on empty required fields at pristine

[AlanCrevon]

Bug, feature request, or proposal:

Request

What is the expected behavior?

.mat-input-invalid class should be added to empty required fields on pristine.

What is the current behavior?

.mat-input-invalid is added only if the field has validator and has been touched.

What is the use-case or motivation for changing an existing behavior?

Empty mandatory fields should be visible at first sight.

Which versions of Angular, Material, OS, browsers are affected?

Angular 4.0.1
Material2 Beta 3

Is there anything else we should know?

I love you. ;)

[jelbourn]

For this we follow the material design spec, which calls for

Show errors for incompatible values during or after a user interacts with a text field.

[AlanCrevon]

Ho... i see. Ok thanks for the answer. :)

[leblancmeneses]

Is there plan to add md-is-error angular/material#1633 ?

There are some cases I want to show an error even if it is untouched.
Example:
Expense: input[type="number"]
Expense itemization: textarea where [required]="selectedEvent.miscExpenses && selectedEvent.miscExpenses > 0"

The user doesn't know that there is a problem with the form. I would like to force a visual with md-is-error.

Otherwise, I have to begin adding tooltips to explain to the user why the button is disabled.

[willshowell]

@leblancmeneses #4750 should accomplish what you need

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