Single-value autocomplete widgets don't warn about field cardinality, lose data silently

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This could use an issue summary update. Mainly what is the proposed solution to solve the issue? I see steps to reproduce but what was the solution decided upon? WIll help committers later?

Added detail to IS.

Can someone verify if this is still an issue?

Just tested in Drupal 10.1 with a standard install
Changed article tags field to cardinality of 1
Created an article by adding TestA then TestB waiting for the autocomplete icon to go away each time
When I clicked save I get

Tags: this field cannot hold more than 1 values.

I also just tested on 10.0.3 with a standard install, similar details as smustgrave, and I also get the "Tags: this field cannot hold more than 1 values." message when trying to save the article. So, perhaps this is fixed in Drupal 10 which is great to hear.

Can someone confirm that there are tests for this behavior in 10.x? If not, I can at least reroll for the added test coverage.

From what I can tell searching for "hold more than" there are instances of test coverage. But this constraint in fieldItemList has been in for 5 years? That doesn't right.

Want to make sure we can move credit for everyone that helped work on this.

Brought this up in the #bugsmash channel and others reported they were able to replicate

From @rkoller

i’ve added an entity reference field to the article content type and picked the reference type basic page. and field settings were limited to 1. and under manage form display i’ve used the default autocomplete option.

and

changed the option from autocompleteto autocomplete (tag style) in manage form display. with that option active i get the limited to 1 warning and i am unable to proceed to save the form. if i switch back to autocomplete i am again able to save more than one tag and just the last is saved as described in the issue summaries steps to reproduce section

Yeah, confirmed it occurs with the regular 'Autocomplete' widget and not the tags style one.

GuyPaddock → changed the visibility of the branch 2796341-entity-reference-field to hidden.