From 3fc7dd185862742ea1c35ad2ffb77d961d1e03ad Mon Sep 17 00:00:00 2001
From: "Yukihiro \"Matz\" Matsumoto" <matz@ruby.or.jp>
Date: Mon, 11 May 2026 14:06:28 +0900
Subject: [PATCH] mruby-proc-ext: Proc#curry should check max arity for lambdas

`lambda {|a, b=nil|}.curry(3)` used to silently return a curried Proc;
CRuby raises `ArgumentError` because the lambda accepts at most 2 args.
Use `self.parameters` to compute the upper bound (count `:req`/`:opt`
entries, unbounded if `:rest`/`:keyrest` is present) and add the
corresponding range check alongside the existing minimum check.

close #2855

Co-authored-by: Claude <noreply@anthropic.com>
---
 mrbgems/mruby-proc-ext/mrblib/proc.rb | 19 ++++++++++++++++---
 mrbgems/mruby-proc-ext/test/proc.rb   |  7 +++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/mrbgems/mruby-proc-ext/mrblib/proc.rb b/mrbgems/mruby-proc-ext/mrblib/proc.rb
index 4e7dfac57..320549ce7 100644
--- a/mrbgems/mruby-proc-ext/mrblib/proc.rb
+++ b/mrbgems/mruby-proc-ext/mrblib/proc.rb
@@ -75,9 +75,22 @@ class Proc
     if lambda?
       type = :lambda
       self_arity = self.arity
-      if (self_arity >= 0 && arity != self_arity) ||
-         (self_arity < 0 && abs[self_arity] > arity)
-        raise ArgumentError, "wrong number of arguments (given #{arity}, expected #{abs[self_arity]})"
+      min_req = abs[self_arity]
+      if self_arity < 0
+        max_arity = 0
+        has_rest = false
+        self.parameters.each do |p|
+          case p[0]
+          when :rest, :keyrest then has_rest = true
+          when :req, :opt then max_arity += 1
+          end
+        end
+        if arity < min_req || (!has_rest && arity > max_arity)
+          expected = (!has_rest && max_arity != min_req) ? "#{min_req}..#{max_arity}" : min_req.to_s
+          raise ArgumentError, "wrong number of arguments (given #{arity}, expected #{expected})"
+        end
+      elsif arity != self_arity
+        raise ArgumentError, "wrong number of arguments (given #{arity}, expected #{self_arity})"
       end
     end
 
diff --git a/mrbgems/mruby-proc-ext/test/proc.rb b/mrbgems/mruby-proc-ext/test/proc.rb
index 7a105323a..fcbeb4962 100644
--- a/mrbgems/mruby-proc-ext/test/proc.rb
+++ b/mrbgems/mruby-proc-ext/test/proc.rb
@@ -64,6 +64,13 @@ assert('Proc#curry') do
 
   assert_false(proc{}.curry.lambda?)
   assert_true(lambda{}.curry.lambda?)
+
+  # #2855: lambda with optional param: curry must validate against max arity
+  l = lambda {|a, b=nil|}
+  assert_raise(ArgumentError) { l.curry(3) }   # over max
+  assert_kind_of Proc, l.curry(2)              # within range
+  assert_kind_of Proc, l.curry(1)              # at min
+  assert_kind_of Proc, lambda {|a, *b|}.curry(99) # rest -> unbounded
 end
 
 assert('Proc#parameters') do