Issue #21563 has been updated by jeremyevans0 (Jeremy Evans). If we want to change the behavior in this case, we should report both the actual argument and the return value of `to_proc`. That's what we do for `*a` where `a.to_a` returns non-Array and for `**h` where `h.to_hash` returns non-Hash): ```ruby A = Class.new a = A.new def a.to_a = 1 def a.to_hash = 1 p(*a) # can't convert A to Array (A#to_a gives Integer) (TypeError) p(**a) # can't convert A to Hash (A#to_hash gives Integer) (TypeError) def a.to_proc = 1 p(&a) # currently: wrong argument type A (expected Proc) (TypeError) # preferable: can't convert A to Proc (A#to_proc gives Integer) (TypeError) ``` ---------------------------------------- Bug #21563: Misleading error message when `to_proc` does not return a Proc in an Object used as a &block argument https://bugs.ruby-lang.org/issues/21563#change-114512 * Author: soulcutter (Bradley Schaefer) * Status: Open * ruby -v: ruby 3.4.5 (2025-07-16 revision 20cda200d3) +PRISM [arm64-darwin24] * Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- When a class implements `#to_proc` it should always return a Proc. Nonetheless bugs are possible, and when an implementation returns something else, and is used as a block argument to a method, you get an error that claims that the argument type is the implementing class rather than the type that was returned from `to_proc`. Reproduction ``` class SayHi def call = "hi" def to_proc = "obviously not a proc" end def callablock(&block) = block.call callablock &SayHi.new # wrong argument type SayHi (expected Proc) (TypeError) ``` In real-world code it may not be _this_ obvious that `to_proc` is returning the wrong type, and so I would expect the error would report the type of object returned by `to_proc` so that you have a more-direct path to understanding the misbehavior. In this case I would expect: wrong argument type String (expected Proc) Or maybe something more-descriptive as an error message, but I don't have a concrete suggestion for that. -- https://bugs.ruby-lang.org/