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So we all know it is possible to directly select into a Map like so:

Map<Id, Account> accounts = new Map<Id, Account>([SELECT Id, Name FROM Account]);

Is it possible to do the same thing, using Database.query():

Map<Id, Account> accounts = new Map<Id, Account>(Database.query('SELECT Id, Name FROM Account'));

Sadly the latter doesn't seem to work.

Adding a bit of info here on top of other answers.

This works:

Map<Id, Account> accounts = new Map<Id, Account>([SELECT Id, Name FROM Account]);

Because the return type from the SOQL is that of Account.

This does not work:

Map<Id, Account> accounts = new Map<Id, Account>(Database.query('SELECT Id, Name FROM Account'));

Because the return type of Database.query is always SObject. So unless you cast the return type to the exact object type, it won't work.

Working versions.

Use SObject in your declaration

Map<Id, SObject> accounts = new Map<Id, SObject>(Database.query('SELECT Id, Name FROM Account'));

OR

As in other answers, cast it to list/array of account:

Map<Id, Account> accounts = new Map<Id, Account>((Account)Database.query('SELECT Id, Name FROM Account'));

It could be that this is one scenario where the "magic casting" of Database.query doesn't work quite right.

How about:

Map<Id, Account> accounts = new Map<Id, Account>((List<Account>)Database.query('SELECT Id, Name FROM Account'));

Any luck?

If you cast it to a List of the expected sObject type first it will work.

E.g.

Map<Id, Account> accounts = 

    new Map<Id, Account>((List<Account>)Database.query('SELECT Id, Name FROM Account'));