From https://www.w3.org/TR/rdf11-concepts/#section-Graph-Literal

The literal value associated with a literal is:

1. If the literal is a language-tagged string, then the literal value is a pair consisting of its lexical form and its language tag, in that order.
2. If the literal's datatype IRI is in the set of recognized datatype IRIs, let d be the referent of the datatype IRI.
   • (a) If the literal's lexical form is in the lexical space of d, then the literal value is the result of applying the lexical-to-value mapping of d to the lexical form.
   • (b) Otherwise, the literal is ill-typed and no literal value can be associated with the literal. Such a case produces a semantic inconsistency but is not syntactically ill-formed. Implementations must accept ill-typed literals and produce RDF graphs from them. Implementations may produce warnings when encountering ill-typed literals.
3. If the literal's datatype IRI is not in the set of recognized datatype IRIs, then the literal value is not defined by this specification.

Given that Literal.ill_formed is for 2b - I think it may be slightly more clear if it was Literal.ill_typed instead of Literal.ill_formed