TYPO3 Core

There is a false claim in [1], saying:

"the default icon is the one defined with the iconfile value."

This is false:

\TYPO3\CMS\Core\Imaging\IconFactory::mapRecordTypeToIconIdentifier() skips 'tcarecords-' . $table . '-default' altogether, when isset($GLOBALS['TCA'][$table]['ctrl']['typeicon_column']) evaluates to true.

Therefore, typeicon_column must always contain a default key mapping to an existing icon (which means you need to duplicate iconfile, but this time as icon identifier rather than as file path).
iconfile is not even considered (e.g. in the List > New Record view) for typeicon_column-enabled tables.

You can choose if this is a code bug in \TYPO3\CMS\Core\Imaging\IconFactory::mapRecordTypeToIconIdentifier() or a documentation bug in [1].

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[1] https://docs.typo3.org/typo3cms/TCAReference/Ctrl/#ctrl-reference-typeicon-column