Field calculator output preview should use selected type

[qgib]

Author Name: Johannes Kroeger (Johannes Kroeger)
Original Redmine Issue: 13672

Redmine category:unknown

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The Field Calculator's "Output preview" should show the same format as using the currently selected "Output field type" would result in.

For example if "Output field type" is set to "Whole number (integer)" it should display a "7", not a "7.123456".

This would at least me make less accidental integer columns when I wanted decimals.

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• easy_fix was configured as 0

[qgib]

Author Name: Jürgen Fischer (@jef-n)

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• category_id was configured as Unknown

[kannes]

Haha, hello me from 2015.

This obviously also affects total conversion failures, e. g. when the user has an expression that returns strings and uses that on a integer column. They will get all NULLs and wonder what went wrong.

Thinking about this now I am not sure if the preview should actually cast to the destination type or if the dialog should rather detect a mismatch and complain. The preview previews the result of the expression and there the actual return value should be shown. The actual problem arises later, when it is cast into the destination type.

And then you might have an expression that actually returns the correct type for some features and not for others. Phew...

I think, a much better solution to the issue would be warnings after population of the column if a type cast changed a value. But that is probably something far in the providers' functionality and not easily accessible, right?