If #to_proc is defined, this uses the following error message format, matching the error message format used for * when to_a returns non-Array and for ** when to_hash returns non-Hash: ``` can't convert ClassName to Proc (ClassName#to_proc gives OtherClassName) ``` If #to_proc is not defined, this uses the following error message format, matching the error message format used when ** is called on a non-Hash not implementing to_hash. ``` no implicit conversion of ClassName into Proc ``` There isn't a similar error for * when called on a non-Array not implementing to_a, as Ruby does not raise for that case. Fixes [Bug #21563]