Improve handling of `?`-variables #1938

Job	Run time
Formatting and Style	21s
Nix / All Tests (self-hosted, linux, normal, Ubuntu-22.04, x86_64-linux)	8m 51s
Nix / All Tests (MacM1, self-macos-12, aarch64-darwin)	6m 47s
Stack / Unit Tests	1m 42s
Nix / All Tests (macos-12, macos-12, x86_64-darwin)	22m 51s
	40m 32s

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