Issue 19842 - only class member functions must be used be used as slots

[dukc]

This is #8137, this time correctly targeting stable.

EDIT: Nope. base commit correct but Pull request target branch still wrong. Third attempt here: #8140

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	19842	critical	std.signals Segfault when connecting with a lambda

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8139"