Bug: yielding in '__close' mess up number of returns

Yielding in a __close metamethod called when returning vararg results
changes the top and so messes up the number of returned values.

testes/locals.lua

@@ -813,6 +813,65 @@ do
 end
 
 
+do
+  -- yielding inside closing metamethods while returning
+  -- (bug in 5.4.3)
+
+  local extrares    -- result from extra yield (if any)
+
+  local function check (body, extra, ...)
+    local t = table.pack(...)   -- expected returns
+    local co = coroutine.wrap(body)
+    if extra then
+      extrares = co()    -- runs until first (extra) yield
+    end
+    local res = table.pack(co())   -- runs until yield inside '__close'
+    assert(res.n == 2 and res[2] == nil)
+    local res2 = table.pack(co())   -- runs until end of function
+    assert(res2.n == t.n)
+    for i = 1, #t do
+      if t[i] == "x" then
+        assert(res2[i] == res[1])    -- value that was closed
+      else
+        assert(res2[i] == t[i])
+      end
+    end
+  end
+
+  local function foo ()
+    local x <close> = func2close(coroutine.yield)
+    local extra <close> = func2close(function (self)
+      assert(self == extrares)
+      coroutine.yield(100)
+    end)
+    extrares = extra
+    return table.unpack{10, x, 30}
+  end
+  check(foo, true, 10, "x", 30)
+  assert(extrares == 100)
+
+  local function foo ()
+    local x <close> = func2close(coroutine.yield)
+    return
+  end
+  check(foo, false)
+
+  local function foo ()
+    local x <close> = func2close(coroutine.yield)
+    local y, z = 20, 30
+    return x
+  end
+  check(foo, false, "x")
+
+  local function foo ()
+    local x <close> = func2close(coroutine.yield)
+    local extra <close> = func2close(coroutine.yield)
+    return table.unpack({}, 1, 100)   -- 100 nils
+  end
+  check(foo, true, table.unpack({}, 1, 100))
+
+end
+
 do
   -- yielding inside closing metamethods after an error