codegen.c: raise NoMatchingPatternError in case/in without else

case/in without else clause now raises NoMatchingPatternError
when no pattern matches, matching CRuby behavior. Fixes #6741.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-compiler/core/codegen.c

@@ -4455,10 +4455,9 @@ codegen_case_match(codegen_scope *s, node *varnode, int val)
     current_in = current_in->cdr;
   }
 
-  /* No pattern matched - generate nil or error */
-  if (val) {
-    genop_1(s, OP_LOADNIL, cursp());
-  }
+  /* No pattern matched - raise NoMatchingPatternError */
+  genop_1(s, OP_LOADFALSE, cursp());
+  genop_1(s, OP_MATCHERR, cursp());
 
   /* Dispatch all end jumps */
   if (case_end_jumps != JMPLINK_START) {

test/t/syntax.rb

@@ -1246,12 +1246,13 @@ assert('pattern matching - NoMatchingPatternError') do
     assert_true e.message.is_a?(String)
   end
 
-  # case/in without else returns nil (no match)
-  result = case 5
-  in 1 then :one
-  in 2 then :two
+  # case/in without else raises NoMatchingPatternError
+  assert_raise(NoMatchingPatternError) do
+    case 5
+    in 1 then :one
+    in 2 then :two
+    end
   end
-  assert_nil result
 end
 
 assert('pattern matching - complex patterns') do