#30548: Improve exception message when BaseExpression raises "'Expression 
mixed types. You must set output_field.'"
               Reporter:  Keryn      |          Owner:  nobody
  Knight                             |
                   Type:             |         Status:  new
  Cleanup/optimization               |
              Component:  Database   |        Version:  master
  layer (models, ORM)                |
               Severity:  Normal     |       Keywords:  expressions orm
           Triage Stage:             |      Has patch:  0
  Unreviewed                         |
    Needs documentation:  0          |    Needs tests:  0
Patch needs improvement:  0          |  Easy pickings:  0
                  UI/UX:  0          |
 source which raises the exception] has enough information to say both what
 types were found, and which of those were unexpected, **and** probably
 have a useful `repr()`

 In the test suite, the unexpected output types encountered seem to be
 `DurationField` and `IntegerField`, so a more thorough message might be
 something like:
 `Expression repr(self) contained mixed types: DateField, DurationField.
 DurationField was unexpected; you must set the output_field= for this
 Expression to either DurationField(), DateField() or ...` (''??? I dunno,
 some concrete explanation of what the output_field has to be/implement if
 you're not going to use any of the builtins'')

 The merit of including the repr is arguable, as the `Expression` may not
 what the user put in (eg: in the test suite it always seems to be a
 `CombinedExpression(lhs, connector, rhs)`) but it gives more of a hint in
 a query which contains multiple expressions (either nested or separate) as
 to which one is actually causing the problem vs just being told "something
 was wrong. Put an output_field= everywhere until it ceases, your guess is
 as good as mine"; at the very least the word Expression could be replaced
 with the class name which is actually raising it.

Ticket URL: <https://code.djangoproject.com/ticket/30548>
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