If a constraint type is only supported for certain context types (e.g. “mandatory qualifiers” can only be checked on the main snak, not on qualifiers and references), but the constraint scope in the constraint parameters specifies a different context type (e.g. “constraint checked on qualifiers”), we show a warning on the constraint definition –

– but when checking the constraint, we ignore the supported context types that the constraint type declares (main snak only), and call the constraint checker on contexts that it doesn’t support. (In the case of the “mandatory qualifiers” checker, this results in a crash.)