Fix `super` from aliased methods to work correctly; fix #4718

We needed to preserve the original method name somewhere. We kept it in
the `env` structure pointed from aliased methods. #1457 and #1531 tried
to address this issue. But this patch is more memory efficient.

Limitation: this fix does not support `super` from methods defined by
`define_method`. This limitation may be addressed in the future, but
it's low priority.

src/class.c

@@ -1741,6 +1741,25 @@ mrb_alias_method(mrb_state *mrb, struct RClass *c, mrb_sym a, mrb_sym b)
 {
   mrb_method_t m = mrb_method_search(mrb, c, b);
 
+  if (!MRB_METHOD_CFUNC_P(m)) {
+    struct RProc *p = MRB_METHOD_PROC(m);
+
+    if (MRB_PROC_ENV_P(p)) {
+      MRB_PROC_ENV(p)->mid = b;
+    }
+    else {
+      struct RClass *tc = MRB_PROC_TARGET_CLASS(p);
+      struct REnv *e = (struct REnv*)mrb_obj_alloc(mrb, MRB_TT_ENV, NULL);
+
+      e->mid = b;
+      if (tc) {
+        e->c = tc;
+        mrb_field_write_barrier(mrb, (struct RBasic*)e, (struct RBasic*)tc);
+      }
+      p->e.env = e;
+      p->flags |= MRB_PROC_ENVSET;
+    }
+  }
   mrb_define_method_raw(mrb, c, a, m);
 }
 

src/proc.c

@@ -77,6 +77,9 @@ closure_setup(mrb_state *mrb, struct RProc *p)
       e->c = tc;
       mrb_field_write_barrier(mrb, (struct RBasic*)e, (struct RBasic*)tc);
     }
+    if (MRB_PROC_ENV_P(up) && MRB_PROC_ENV(up)->cxt == NULL) {
+      e->mid = MRB_PROC_ENV(up)->mid;
+    }
   }
   if (e) {
     p->e.env = e;

src/vm.c

@@ -1558,9 +1558,13 @@ RETRY_TRY_BLOCK:
       struct RClass *cls;
       mrb_callinfo *ci = mrb->c->ci;
       mrb_value recv, blk;
+      struct RProc *p = ci->proc;
       mrb_sym mid = ci->mid;
-      struct RClass* target_class = MRB_PROC_TARGET_CLASS(ci->proc);
+      struct RClass* target_class = MRB_PROC_TARGET_CLASS(p);
 
+      if (MRB_PROC_ENV_P(p) && p->e.env->mid && p->e.env->mid != mid) { /* alias support */
+        mid = p->e.env->mid;    /* restore old mid */
+      }
       mrb_assert(bidx < irep->nregs);
 
       if (mid == 0 || !target_class) {
@@ -2020,7 +2024,7 @@ RETRY_TRY_BLOCK:
             if (MRB_PROC_ENV_P(dst)) {
               struct REnv *e = MRB_PROC_ENV(dst);
 
-              if (!MRB_ENV_STACK_SHARED_P(e) || e->cxt != mrb->c) {
+              if (!MRB_ENV_STACK_SHARED_P(e) || (e->cxt && e->cxt != mrb->c)) {
                 localjump_error(mrb, LOCALJUMP_ERROR_RETURN);
                 goto L_RAISE;
               }