False negative for valid dereference of single element collection

This was raised here:

https://salesforce.stackexchange.com/questions/410161/assignment-of-type-mapid-sobject-values-property-to-object

and I’m trying to generalize it a bit as follows:

public with sharing class SingleElementReferenceTest {
    public static void testSingleSobjectElementReference() {
        Map<Id, Account> accountsById = new Map<Id, Account>([SELECT Id, AccountNumber FROM Account LIMIT 1]);
        // This should work but can lead to errors if there's not exactly one account in the map
        System.debug(accountsById.values().AccountNumber);

        // This is also fine, so the key type doesn't seem to matter
        Map<Datetime, Account> accountsByName = new Map<Datetime, Account>();
        Account account = [SELECT Name, AccountNumber, LastModifiedDate FROM Account LIMIT 1];
        accountsByName.put(account.LastModifiedDate, account);
        System.debug(accountsByName.values().AccountNumber);
    }

    public class InnerValueObject {
        public String myProperty = UserInfo.getUserName();
    }

    public static void testSingleElementReference() {
        Map<Id, InnerValueObject> innerValueObjectMap = new Map<Id, InnerValueObject>();
        innerValueObjectMap.put(UserInfo.getOrganizationId(), new InnerValueObject());

        // This compiles but GACKs, presumably because the map value type is not an SObject type
        System.debug(innerValueObjectMap.values().myProperty);

        // This does not compile
        List<InnerValueObject> innerValueObjects = innerValueObjectMap.values();
        System.debug(innerValueObjects.myProperty);

        // Nor does this
        List<InnerValueObject> innerValueObjectList = new List<InnerValueObject>();
        System.debug(innerValueObjectList.myProperty);

        // Nor does this
        InnerValueObject[] innerValueObjectArray = new InnerValueObject[] { new InnerValueObject() };
        System.debug(innerValueObjectArray.myProperty);

        // Nor does this
        Set<InnerValueObject> innerValueObjectSet = new Set<InnerValueObject>();
        System.debug(innerValueObjectSet.myProperty);
    }
}

As is likely evident from the example above, it’s difficult to tell exactly when this should be considered valid and when it shouldn’t be, though as stated in the linked SE Q&A above, it seems like Map.values() specifically should be handled in a similar fashion to a query result and allowed to evaluate to a list or single instance of the contained data (not just SObject).

This is one of those where I’m torn between supporting the idiom for “correctness” vs. flagging it to force users to think about the potential pitfalls of using it. I think that perhaps the right solution is to update IC2’s expression type evaluator to allow it, but at the same time to add a code inspection to flag it as a dangerous idiom as there’s no way of knowing at compile-time whether the returned collection has exactly one element.

Once again, Apex has some weird behaviors sometimes…

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