has_content
It is true if the variable exists (and isn't Java null) and is not "empty", otherwise it is false. The meaning of "empty" depends on the concrete case. This follows intuitive common-sense ideas. The following are empty: a string with 0 length, a markup output value with 0 length markup, a sequence or hash with no sub variables, a collection which has passed the last element. If the value is not of any of these types, then it counts as non-empty if it's a number or a date or a boolean (e.g. 0 and false are not empty), otherwise it counts as empty. Note that when your data-model implements multiple template model interfaces you may get unexpected results. However, when in doubt you can use always use expr!?size > 0 or expr!?length > 0 instead of expr?has_content.
This buit-in is exceptional in that you can use the parentheses trick like with the default value operator. That is, you can write bothproduct.color?has_content and (product.color)?has_content. The first doesn't handle the case when product is missing, the last does.