Comment by bmc7505

Just because the current implementation of the Idris type checker always terminates due to the size change termination heuristic does not mean that type checking Idris is decidable. If the type system as specified allows an author to express undecidable statements, it should not be considered decidable, regardless of whether one type checking implementation terminates on all inputs. If some type checker does not implement its type system, this is a property of the implementation, not the language. Otherwise any undecidable type system could easily call itself decidable by checking only a subset of the specification (in fact, this is exactly how most checkers for undecidable type systems work anyway). If the type system for a language is undecidable, type checking that language is undecidable.