Struct setter's parameters == [:req, :_]

fix [Bug #18405]

Note that the parameter name `_` is not a spec, so we shouldn't
rely on this behavior.

proc.c

@@ -2910,6 +2910,69 @@ rb_method_location(VALUE method)
     return method_def_location(rb_method_def(method));
 }
 
+static const rb_method_definition_t *
+vm_proc_method_def(VALUE procval)
+{
+    const rb_proc_t *proc;
+    const struct rb_block *block;
+    const struct vm_ifunc *ifunc;
+
+    GetProcPtr(procval, proc);
+    block = &proc->block;
+
+    if (vm_block_type(block) == block_type_ifunc &&
+        IS_METHOD_PROC_IFUNC(ifunc = block->as.captured.code.ifunc)) {
+        return rb_method_def((VALUE)ifunc->data);
+    }
+    else {
+        return NULL;
+    }
+}
+
+static VALUE
+method_def_parameters(const rb_method_definition_t *def)
+{
+    const rb_iseq_t *iseq;
+    const rb_method_definition_t *bmethod_def;
+
+    switch (def->type) {
+      case VM_METHOD_TYPE_ISEQ:
+        iseq = method_def_iseq(def);
+        return rb_iseq_parameters(iseq, 0);
+      case VM_METHOD_TYPE_BMETHOD:
+        if ((iseq = method_def_iseq(def)) != NULL) {
+            return rb_iseq_parameters(iseq, 0);
+        }
+        else if ((bmethod_def = vm_proc_method_def(def->body.bmethod.proc)) != NULL) {
+            return method_def_parameters(bmethod_def);
+        }
+        break;
+
+      case VM_METHOD_TYPE_ALIAS:
+        return method_def_parameters(def->body.alias.original_me->def);
+
+      case VM_METHOD_TYPE_OPTIMIZED:
+        if (def->body.optimized.type == OPTIMIZED_METHOD_TYPE_STRUCT_ASET) {
+            VALUE param = rb_ary_new_from_args(2, ID2SYM(rb_intern("req")), ID2SYM(rb_intern("_")));
+            return rb_ary_new_from_args(1, param);
+        }
+        break;
+
+      case VM_METHOD_TYPE_CFUNC:
+      case VM_METHOD_TYPE_ATTRSET:
+      case VM_METHOD_TYPE_IVAR:
+      case VM_METHOD_TYPE_ZSUPER:
+      case VM_METHOD_TYPE_UNDEF:
+      case VM_METHOD_TYPE_NOTIMPLEMENTED:
+      case VM_METHOD_TYPE_MISSING:
+      case VM_METHOD_TYPE_REFINED:
+        break;
+    }
+
+    return rb_unnamed_parameters(method_def_aritry(def));
+
+}
+
 /*
  * call-seq:
  *    meth.parameters  -> array
@@ -2932,11 +2995,7 @@ rb_method_location(VALUE method)
 static VALUE
 rb_method_parameters(VALUE method)
 {
-    const rb_iseq_t *iseq = rb_method_iseq(method);
+    return method_def_parameters(rb_method_def(method));
-    if (!iseq) {
-	return rb_unnamed_parameters(method_arity(method));
-    }
-    return rb_iseq_parameters(iseq, 0);
 }
 
 /*

test/ruby/test_struct.rb

@@ -497,6 +497,21 @@ module TestStruct
     assert_equal(42, x.public_send("a"))
   end
 
+  def test_parameters
+    klass = @Struct.new(:a)
+    assert_equal [], klass.instance_method(:a).parameters
+    # NOTE: :_ may not be a spec.
+    assert_equal [[:req, :_]], klass.instance_method(:a=).parameters
+
+    klass.module_eval do
+      define_method(:b=, &klass.new.method(:a=).to_proc)
+      alias c= a=
+    end
+
+    assert_equal [[:req, :_]], klass.instance_method(:b=).parameters
+    assert_equal [[:req, :_]], klass.instance_method(:c=).parameters
+  end
+
   class TopStruct < Test::Unit::TestCase
     include TestStruct