eval: fix assertion failure in garbage collection (#12436)

* eval: fix assertion failure in garbage collection

fixes #12387, #12430

Lists with CopyID+1 linked only from previous_funccal may be removed in
the garbage collection. Therefore, the terms of the assertions are not
correct. This can be confirmed by the following (The l:x with CopyID+1 of
the first function call needs to be removed by garbage collection):

func! s:f()
  let l:x = [1]
  let g:x = l:
endfunc

for _ in range(2)
  call s:f()
endfor
call garbagecollect()
" press any key

* test: add test for #12387, #12430

1 files changed, 15 insertions, 0 deletions

diff --git a/test/functional/eval/let_spec.lua b/test/functional/eval/let_spec.lua
index f8fcdfd41f..5bc703b567 100644
--- a/test/functional/eval/let_spec.lua
+++ b/test/functional/eval/let_spec.lua
@@ -75,4 +75,19 @@ describe(':let', function()
     command(cmd_get_child_env)
     eq(eval('$NVIM_TEST'), eval('g:env_from_child'))
   end)
+
+  it("release of list assigned to l: variable does not trigger assertion #12387, #12430", function()
+    source([[
+      func! s:f()
+        let l:x = [1]
+        let g:x = l:
+      endfunc
+      for _ in range(2)
+        call s:f()
+      endfor
+      call garbagecollect()
+      call feedkeys('i', 't')
+    ]])
+    eq(1, eval('1'))
+  end)
 end)