When a product is deleted, its product code is freed but not its variant product code.

Step to reproduce on a clean install:

• I create an attribute groupe.
• I create an attribute "Selection" in previous attribute group and set two options.
• I create a product type, drag-n-drop the attribute group in the Attributes field, set the attribute "Used for product variants" to "on", leave the default for "Field definitions" and optional fields.
• I create a product of this type.
• I generate its variants with the "Generate variants" button for the two options.
• I delete the product.
• I recreate the same product (at least a product with the same product code).
• I generate the variants: Error “SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '<product_code>-1' for key 'ibexa_product_code_uidx'“.

Same things happen with manually creating a variant instead using the generator. If a code has been used for a variant, deleting the product won't make the code available for variant recreation.