Fix incorrect use of VM_CALL_KW_SPLAT_MUT in zsuper with keyword splat

For zsuper calls with a keyword splat but no actual keywords, the
keyword splat is passed directly, so it cannot be mutable, because
if the callee accepts a keyword splat, changes to the keyword splat
by the callee would be reflected in the caller.

While here, simplify the logic when the method supports
literal keywords.  I don't think it is possible for
a method with has_kw param flags to not have keywords, so add an
assertion for that, and set VM_CALL_KW_SPLAT_MUT in a single place.

compile.c

@@ -9534,14 +9534,11 @@ compile_super(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *const node, i
             if (local_body->param.flags.has_kwrest) {
                 int idx = local_body->local_table_size - local_kwd->rest_start;
                 ADD_GETLOCAL(args, node, idx, lvar_level);
-                if (local_kwd->num > 0) {
+                assert(local_kwd->num > 0);
                 ADD_SEND (args, node, rb_intern("dup"), INT2FIX(0));
-                    flag |= VM_CALL_KW_SPLAT_MUT;
-                }
             }
             else {
                 ADD_INSN1(args, node, newhash, INT2FIX(0));
-                flag |= VM_CALL_KW_SPLAT_MUT;
             }
             for (i = 0; i < local_kwd->num; ++i) {
                 ID id = local_kwd->table[i];
@@ -9550,13 +9547,13 @@ compile_super(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *const node, i
                 ADD_GETLOCAL(args, node, idx, lvar_level);
             }
             ADD_SEND(args, node, id_core_hash_merge_ptr, INT2FIX(i * 2 + 1));
-            flag |= VM_CALL_KW_SPLAT;
+            flag |= VM_CALL_KW_SPLAT| VM_CALL_KW_SPLAT_MUT;
         }
         else if (local_body->param.flags.has_kwrest) {
             int idx = local_body->local_table_size - local_kwd->rest_start;
             ADD_GETLOCAL(args, node, idx, lvar_level);
             argc++;
-            flag |= VM_CALL_KW_SPLAT | VM_CALL_KW_SPLAT_MUT;
+            flag |= VM_CALL_KW_SPLAT;
         }
     }
 

test/ruby/test_super.rb

@@ -558,6 +558,18 @@ class TestSuper < Test::Unit::TestCase
     end
   end
 
+  def test_zsuper_kw_splat_not_mutable
+    extend(Module.new{def a(**k) k[:a] = 1 end})
+    extend(Module.new do
+      def a(**k)
+        before = k.dup
+        super
+        [before, k]
+      end
+    end)
+    assert_equal(*a)
+  end
+
   def test_from_eval
     bug10263 = '[ruby-core:65122] [Bug #10263a]'
     a = Class.new do