YJIT: Fix missing arity check for splat calls to methods with optionals

Previously, for splat callsites that land in methods with optional
parameters, we didn't reject the case where the caller supplies too many
arguments. Accepting those calls previously caused YJIT to construct
corrupted control frames, which leads to crashes if the callee uses
certain stack walking methods such as Kernel#raise and String#gsub (for
setting up the frame-local `$~`).

Example crash in a debug build:

    Assertion Failed: ../vm_core.h:1375:VM_ENV_FLAGS:FIXNUM_P(flags)

bootstraptest/test_yjit.rb

@@ -1,3 +1,29 @@
+# regression test for arity check with splat
+assert_equal '[:ae, :ae]', %q{
+  def req_one(a_, b_ = 1) = raise
+
+  def test(args)
+    req_one *args
+  rescue ArgumentError
+    :ae
+  end
+
+  [test(Array.new 5), test([])]
+}
+
+# regression test for arity check with splat and send
+assert_equal '[:ae, :ae]', %q{
+  def two_reqs(a, b_, _ = 1) = a.gsub(a, a)
+
+  def test(name, args)
+    send(name, *args)
+  rescue ArgumentError
+    :ae
+  end
+
+  [test(:two_reqs, ["g", nil, nil, nil]), test(:two_reqs, ["g"])]
+}
+
 # regression test for GC marking stubs in invalidated code
 assert_normal_exit %q{
   garbage = Array.new(10_000) { [] } # create garbage to cause iseq movement

yjit/src/codegen.rs

@@ -6116,9 +6116,14 @@ fn gen_send_iseq(
             unsafe { rb_yjit_array_len(array) as u32}
         };
 
-        if opt_num == 0 && required_num != array_length as i32 + argc - 1 && !iseq_has_rest {
-            gen_counter_incr(asm, Counter::send_iseq_splat_arity_error);
-            return None;
+        // Arity check accounting for size of the splat. When callee has rest parameters, we insert
+        // runtime guards later in copy_splat_args_for_rest_callee()
+        if !iseq_has_rest {
+            let supplying = argc - 1 + array_length as i32;
+            if (required_num..=required_num + opt_num).contains(&supplying) == false {
+                gen_counter_incr(asm, Counter::send_iseq_splat_arity_error);
+                return None;
+            }
         }
 
         if iseq_has_rest && opt_num > 0 {