id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
35257	db.models.expressions _connector_combinations for Numeric with NULL returns only FloatField instead of (IntegerField, DecimalField, FloatField)	Sharon Woo	Sharon Woo	"Discovered via writing tests for this related ticket with a LOT of help from David Sanders -- https://code.djangoproject.com/ticket/35235.

In https://github.com/django/django/blob/ef2434f8508551fee183079ab471b1dc325c7acb/django/db/models/expressions.py#L599-L613 via https://github.com/django/django/pull/15271,

the dictionary comprehension below

{{{
 {
        connector: [
            (field_type, NoneType, field_type),
            (NoneType, field_type, field_type),
        ]
        for connector in (
            Combinable.ADD,
            Combinable.SUB,
            Combinable.MUL,
            Combinable.DIV,
            Combinable.MOD,
            Combinable.POW,
        )
        for field_type in (fields.IntegerField, fields.DecimalField, fields.FloatField)
    },
}}}

returns 

{{{
Combinable.ADD: [(fields.FloatField, NoneType, fields.FloatField),
                     (NoneType, fields.FloatField, fields.FloatField),],
Combinable.SUB: [(fields.FloatField, NoneType, fields.FloatField),
                     (NoneType, fields.FloatField, fields.FloatField),],
...
}}}

instead of the expected 

{{{
Combinable.ADD: [(fields.IntegerField, NoneType, fields.IntegerField),
                     (NoneType, fields.IntegerField, fields.IntegerField),
                     (fields.DecimalField, NoneType, fields.DecimalField),
                     (NoneType, fields.DecimalField, fields.DecimalField),
                     (fields.FloatField, NoneType, fields.FloatField),
                     (NoneType, fields.FloatField, fields.FloatField),
                     ],
...
}}}

This leads to only FloatField throwing FieldError with `_output_field_or_none` when fields are combined with NULL.

Examples you can run right now: 

{{{
original_dict_comp = {
        connector: (
            (field_type, None, field_type),
            (None, field_type, field_type),
        )
        for connector in (
        'ADD', 'SUB', 'MUL', 'DIV', 'MOD', 'POW',
        )
        for field_type in ('IntegerField', 'DecimalField', 'FloatField')
    }

# there are a few ways to write this one
fixed_dict_comp = {
    key: [
        (field_type, None, field_type)
        for field_type in ['IntegerField', 'DecimalField', 'FloatField']
    ] + [
        (None, field_type, field_type)
        for field_type in ['IntegerField', 'DecimalField', 'FloatField']
    ]
    for key in ['ADD', 'SUB', 'MUL', 'DIV', 'MOD', 'POW']
}
}}}
"	Bug	closed	Database layer (models, ORM)	5.0	Normal	fixed			Ready for checkin	1	0	0	0	0	0