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

[dukc]

This is #8137,changed to target stable

[dlang-bot]

Thanks for your pull request and interest in making D better, @dukc! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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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 "stable + phobos#8140"

[RazvanN7]

Buildkite fails with: fatal: Couldn't find remote ref refs/pull/8139/merge . Maybe rebase this?

[RazvanN7]

(rebased it for you)