Always remove empty keyword hashes when calling methods

While doing so is not backwards compatible with Ruby 2.6, it is
necessary for generic argument forwarding to work for all methods:

```ruby
def foo(*args, **kw, &block)
  bar(*args, **kw, &block)
end
```

If you do not remove empty keyword hashes, and bar does not accept
keyword arguments, then a call to foo without keyword arguments
calls bar with an extra positional empty hash argument.

1 files changed, 18 insertions, 13 deletions

diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index baf93de742..4ae94e3fd7 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -1740,7 +1740,7 @@ rb_iseq_only_kwparam_p(const rb_iseq_t *iseq)
 static inline void
 CALLER_SETUP_ARG(struct rb_control_frame_struct *restrict cfp,
                  struct rb_calling_info *restrict calling,
-                 const struct rb_call_info *restrict ci, int remove_empty_keyword_hash)
+                 const struct rb_call_info *restrict ci)
 {
     if (UNLIKELY(IS_ARGS_SPLAT(ci))) {
         /* This expands the rest argument to the stack.
@@ -1755,9 +1755,14 @@ CALLER_SETUP_ARG(struct rb_control_frame_struct *restrict cfp,
          */
         vm_caller_setup_arg_kw(cfp, calling, ci);
     }
-    if (UNLIKELY(calling->kw_splat && remove_empty_keyword_hash)) {
+    if (UNLIKELY(calling->kw_splat)) {
         /* This removes the last Hash object if it is empty.
          * So, ci->flag & VM_CALL_KW_SPLAT is now inconsistent.
+         * However, you can use ci->flag & VM_CALL_KW_SPLAT to
+         * determine whether a hash should be added back with
+         * warning (for backwards compatibility in cases where
+         * the method does not have the number of required
+         * arguments.
          */
         if (RHASH_EMPTY_P(cfp->sp[-1])) {
             cfp->sp--;
@@ -1873,7 +1878,7 @@ vm_callee_setup_arg(rb_execution_context_t *ec, struct rb_calling_info *calling,
     if (LIKELY(!(ci->flag & VM_CALL_KW_SPLAT))) {
         if (LIKELY(rb_simple_iseq_p(iseq))) {
             rb_control_frame_t *cfp = ec->cfp;
-            CALLER_SETUP_ARG(cfp, calling, ci, 0); /* splat arg */
+            CALLER_SETUP_ARG(cfp, calling, ci);
 
             if (calling->argc != iseq->body->param.lead_num) {
                 argument_arity_error(ec, iseq, calling->argc, iseq->body->param.lead_num, iseq->body->param.lead_num);
@@ -1884,7 +1889,7 @@ vm_callee_setup_arg(rb_execution_context_t *ec, struct rb_calling_info *calling,
         }
         else if (rb_iseq_only_optparam_p(iseq)) {
             rb_control_frame_t *cfp = ec->cfp;
-            CALLER_SETUP_ARG(cfp, calling, ci, 0); /* splat arg */
+            CALLER_SETUP_ARG(cfp, calling, ci);
 
             const int lead_num = iseq->body->param.lead_num;
             const int opt_num = iseq->body->param.opt_num;
@@ -2237,7 +2242,7 @@ vm_call_cfunc(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp, struct rb
 {
     RB_DEBUG_COUNTER_INC(ccf_cfunc);
 
-    CALLER_SETUP_ARG(reg_cfp, calling, ci, 0);
+    CALLER_SETUP_ARG(reg_cfp, calling, ci);
     return vm_call_cfunc_with_frame(ec, reg_cfp, calling, ci, cc);
 }
 
@@ -2279,7 +2284,7 @@ vm_call_bmethod(rb_execution_context_t *ec, rb_control_frame_t *cfp, struct rb_c
     VALUE *argv;
     int argc;
 
-    CALLER_SETUP_ARG(cfp, calling, ci, 0);
+    CALLER_SETUP_ARG(cfp, calling, ci);
     argc = calling->argc;
     argv = ALLOCA_N(VALUE, argc);
     MEMCPY(argv, cfp->sp - argc, VALUE, argc);
@@ -2309,7 +2314,7 @@ vm_call_opt_send(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp, struct
     struct rb_call_info_with_kwarg ci_entry;
     struct rb_call_cache cc_entry, *cc;
 
-    CALLER_SETUP_ARG(reg_cfp, calling, orig_ci, 0);
+    CALLER_SETUP_ARG(reg_cfp, calling, orig_ci);
 
     i = calling->argc - 1;
 
@@ -2414,7 +2419,7 @@ vm_call_method_missing(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp,
     struct rb_call_cache cc_entry, *cc;
     unsigned int argc;
 
-    CALLER_SETUP_ARG(reg_cfp, calling, orig_ci, 0);
+    CALLER_SETUP_ARG(reg_cfp, calling, orig_ci);
     argc = calling->argc+1;
 
     ci_entry.flag = VM_CALL_FCALL | VM_CALL_OPT_SEND | (calling->kw_splat ? VM_CALL_KW_SPLAT : 0);
@@ -2619,14 +2624,14 @@ vm_call_method_each_type(rb_execution_context_t *ec, rb_control_frame_t *cfp, st
 	return vm_call_cfunc(ec, cfp, calling, ci, cc);
 
       case VM_METHOD_TYPE_ATTRSET:
-	CALLER_SETUP_ARG(cfp, calling, ci, 1);
+        CALLER_SETUP_ARG(cfp, calling, ci);
 	rb_check_arity(calling->argc, 1, 1);
 	cc->aux.index = 0;
         CC_SET_FASTPATH(cc, vm_call_attrset, !((ci->flag & VM_CALL_ARGS_SPLAT) || (ci->flag & VM_CALL_KWARG)));
 	return vm_call_attrset(ec, cfp, calling, ci, cc);
 
       case VM_METHOD_TYPE_IVAR:
-	CALLER_SETUP_ARG(cfp, calling, ci, 1);
+        CALLER_SETUP_ARG(cfp, calling, ci);
 	rb_check_arity(calling->argc, 0, 0);
 	cc->aux.index = 0;
         CC_SET_FASTPATH(cc, vm_call_ivar, !(ci->flag & VM_CALL_ARGS_SPLAT));
@@ -2927,7 +2932,7 @@ vm_callee_setup_block_arg(rb_execution_context_t *ec, struct rb_calling_info *ca
 	rb_control_frame_t *cfp = ec->cfp;
 	VALUE arg0;
 
-	CALLER_SETUP_ARG(cfp, calling, ci, 0); /* splat arg */
+        CALLER_SETUP_ARG(cfp, calling, ci);
 
         if (calling->kw_splat) {
             rb_warn_keyword_to_last_hash(calling, ci, iseq);
@@ -3015,7 +3020,7 @@ vm_invoke_symbol_block(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp,
 {
     VALUE val;
     int argc;
-    CALLER_SETUP_ARG(ec->cfp, calling, ci, 0);
+    CALLER_SETUP_ARG(ec->cfp, calling, ci);
     argc = calling->argc;
     val = vm_yield_with_symbol(ec, symbol, argc, STACK_ADDR_FROM_TOP(argc), calling->kw_splat, calling->block_handler);
     POPN(argc);
@@ -3029,7 +3034,7 @@ vm_invoke_ifunc_block(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp,
 {
     VALUE val;
     int argc;
-    CALLER_SETUP_ARG(ec->cfp, calling, ci, 0);
+    CALLER_SETUP_ARG(ec->cfp, calling, ci);
     argc = calling->argc;
     val = vm_yield_with_cfunc(ec, captured, captured->self, argc, STACK_ADDR_FROM_TOP(argc), calling->kw_splat, calling->block_handler, NULL);
     POPN(argc); /* TODO: should put before C/yield? */