From: "byroot (Jean Boussier) via ruby-core" <ruby-core@...>
Date: 2026-06-30T22:18:30+00:00
Subject: [ruby-core:125900] [Ruby Feature#22136] `sprintf` shouldn't raise ArgumentError when $DEBUG is set

Issue #22136 has been updated by byroot (Jean Boussier).


I don't think so, the code very explicitly raises in debug mode:

```c
if (RTEST(ruby_debug)) rb_raise(rb_eArgError, "%s", mesg);
```

And that has been the case for at least 18 years: https://github.com/byroot/ruby/commit/549c345cefe03ab3f35b9cca1192a0062f288eca, and speced for about as long: https://github.com/ruby/spec/blame/06fba2498a086886191d05b91f57f1e54bf47e8f/core/string/modulo_spec.rb#L103



----------------------------------------
Feature #22136: `sprintf` shouldn't raise ArgumentError when $DEBUG is set
https://bugs.ruby-lang.org/issues/22136#change-117836

* Author: byroot (Jean Boussier)
* Status: Open
----------------------------------------
### Expected behavior

If I'm tracking down a problem and suspect an error might be swallowed somewhere, I do run my test case with `RUBYOPT="-d"` (or simply set `$DEBUG = true`.

I do expect the program to execute all in the same way than without `-d`, expect for extra output on STDERR.

### Actual behavior

If some code happens to call `sprintf` or `String#%` with too many arguments, then it will raise an error and likely break the program, preventing me from simply running the same code but in debug mode.

### Reproduction

```
$ ruby -e 'sprintf("foo", "bar"); p :ok'
:ok

$ ruby -We 'sprintf("foo", "bar"); p :ok'
-e:1: warning: too many arguments for format string
:ok

$ ruby -dWe 'sprintf("foo", "bar"); p :ok'
Exception 'LoadError' at .../lib/ruby/4.0.0/rubygems.rb:1423 - cannot load such file -- rubygems/defaults/operating_system
Exception 'LoadError' at .../lib/ruby/4.0.0/rubygems.rb:1438 - cannot load such file -- rubygems/defaults/ruby
Exception 'ArgumentError' at -e:1 - too many arguments for format string
Exception 'RuntimeError' at <internal:ast>:96 - cannot get AST for ISEQ compiled by prism
-e:1:in 'Kernel#sprintf': too many arguments for format string (ArgumentError)
	from -e:1:in '<main>'
```

### Bug or Feature?

I looked around the Ruby codebase, as far as I can tell this is the only method behaving this way (aside from its aliases like `String#%`).

So I filed this as a Feature, but really in my opinion it is a bug.

Pull Request: https://github.com/ruby/ruby/pull/17579



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