Semi-complex usage of Variables usually cause this:

#
# Fatal error in ../src/compiler/code-assembler.cc, line 677
# Check failed: i != variable_merges_.end() && i->second.size() == merge_count_.
#

See for example PS1 of https://codereview.chromium.org/1974093003/

In order to workaround that it is necessary either to move the Variable definition to the top or create a trampoline block (see PS2 of the mentioned CL).

(CCing people who also write stubs to be aware of the issue).