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Here's a sample app that shows this issue:
https://github.com/SimpleNexus/simplecallerid/

When you import it and update gradle version as the IDE suggests, it fails to build.

Hi there, thank you for reporting this issue.
This was a problem with the constraint layout library, which used attributes under declare styleables without defining them. This has been fixed in constraint layout version 2.0.0 beta 3. You can read more about the issue in https://issuetracker.google.com/136103084

@3 Hello, I can't use beta 3 and beta 2, because of serious issues that I've reported about.
Here's about beta 3:
https://issuetracker.google.com/issues/143411144

You can workaround the missing attribute issue by *temporarily* adding it locally in your values/attrs.xml: <attr name="flow_horizontalSeparator"/>

@5 Just a single attribute?
But why is it considered missing only when I use this gradle version?

In the linked issue https://buganizer.corp.google.com/issues/136103084 there is a full list of attributes to add, but in comment #1 only one of them is reported, so I this is how to fix that particular one.

The newer version of AGP included a fix to aapt2. The bug in aapt2 was that it accidentally allowed non-explicit definition of attributes, when it shouldn't have. Pre 3.0.0 it was disallowed, but the bug was only noticed recently.

@7 I wrote "..." because it was very long.
If you look at the video, you will see the rest...
Why are those attributes missing on a library? How could the library be made without having errors before publishing it?

It was a bug, that went unnoticed because of a bug in aapt2. Both have been fixed now, but since https://issuetracker.google.com/issues/143411144 is preventing you from updating to the newest version of constraint layout, you can use the workaround mentioned in https://buganizer.corp.google.com/issues/136103084 until that one is fixed.

Add to your res/values/attrs.xml:
<resources>
        <attr name="motionProgress"/>
        <attr name="motionTarget"/>
        <attr name="layout_marginBottom"/>
        <attr name="motionPathRotate"/>
        <attr name="duration"/>
        <attr name="flow_verticalSeparator"/>
        <attr name="flow_horizontalSeparator"/>
        <attr name="waveDecay"/>
</resources>

(beta 3 of constraint layout includes these in its own values file, so then there's no need for adding them locally)

@9 I can't use the workaround on the link you've put, because I can't access this link.
Maybe I should join Google to make it easier? Seems I could be a good QA there ...
:)

I tried to add the attributes you wrote, while updating to com.android.tools.build:gradle:4.0.0-alpha04 .
Seems to fix it, but the same issue exists on this dependency :
https://github.com/JcMinarro/RoundKornerLayouts

I don't get yet how this could happen. Do those libraries have the attributes or not? How can gradle know what's missing?
How could a library use its attributes without declaring them in the resources files, and still be able to be built and used by other apps?
It doesn't make sense. The IDE should show a build error in this case, and even if it succeeds to build for some reason, it should crash...

No?

Whoops, sorry. :) Here's the correct link: https://issuetracker.google.com/136103084

These libraries were build with the older version of AGP that had that bug, so that's why it went unnoticed. Now when someone tries to release a library without these attributes they will get a build error. The IDE is also warning about this now.